Rely on sizeof(typename) rather than sizeof(variable) in pqformat.h.

In each of the pq_writeintN functions, the three uses of sizeof() should
surely all be consistent.  I started out to make them all sizeof(ni),
but on reflection let's make them sizeof(typename) instead.  That's more
like our usual style elsewhere, and it's just barely possible that the
failures buildfarm member hornet has shown since 4c119fbcd went in are
caused by the compiler getting confused about sizeof() a parameter that
it's optimizing away.

In passing, improve a couple of comments.

Discussion: https://postgr.es/m/E1e2RML-0002do-Lc@gemulon.postgresql.org

src/include/libpq/pqformat.h

@@ -35,13 +35,13 @@ extern void pq_sendfloat4(StringInfo buf, float4 f);
 extern void pq_sendfloat8(StringInfo buf, float8 f);
 
 /*
- * Append a int8 to a StringInfo buffer, which already has enough space
+ * Append an int8 to a StringInfo buffer, which already has enough space
  * preallocated.
  *
  * The use of pg_restrict allows the compiler to optimize the code based on
  * the assumption that buf, buf->len, buf->data and *buf->data don't
  * overlap. Without the annotation buf->len etc cannot be kept in a register
- * over subsequent pq_writeint* calls.
+ * over subsequent pq_writeintN calls.
  *
  * The use of StringInfoData * rather than StringInfo is due to MSVC being
  * overly picky and demanding a * before a restrict.
@@ -51,13 +51,13 @@ pq_writeint8(StringInfoData *pg_restrict buf, int8 i)
 {
 	int8		ni = i;
 
-	Assert(buf->len + sizeof(i) <= buf->maxlen);
-	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(ni));
-	buf->len += sizeof(i);
+	Assert(buf->len + sizeof(int8) <= buf->maxlen);
+	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(int8));
+	buf->len += sizeof(int8);
 }
 
 /*
- * Append a int16 to a StringInfo buffer, which already has enough space
+ * Append an int16 to a StringInfo buffer, which already has enough space
  * preallocated.
  */
 static inline void
@@ -65,13 +65,13 @@ pq_writeint16(StringInfoData *pg_restrict buf, int16 i)
 {
 	int16		ni = pg_hton16(i);
 
-	Assert(buf->len + sizeof(ni) <= buf->maxlen);
-	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(i));
-	buf->len += sizeof(i);
+	Assert(buf->len + sizeof(int16) <= buf->maxlen);
+	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(int16));
+	buf->len += sizeof(int16);
 }
 
 /*
- * Append a int32 to a StringInfo buffer, which already has enough space
+ * Append an int32 to a StringInfo buffer, which already has enough space
  * preallocated.
  */
 static inline void
@@ -79,13 +79,13 @@ pq_writeint32(StringInfoData *pg_restrict buf, int32 i)
 {
 	int32		ni = pg_hton32(i);
 
-	Assert(buf->len + sizeof(i) <= buf->maxlen);
-	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(i));
-	buf->len += sizeof(i);
+	Assert(buf->len + sizeof(int32) <= buf->maxlen);
+	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(int32));
+	buf->len += sizeof(int32);
 }
 
 /*
- * Append a int64 to a StringInfo buffer, which already has enough space
+ * Append an int64 to a StringInfo buffer, which already has enough space
  * preallocated.
  */
 static inline void
@@ -93,9 +93,9 @@ pq_writeint64(StringInfoData *pg_restrict buf, int64 i)
 {
 	int64		ni = pg_hton64(i);
 
-	Assert(buf->len + sizeof(i) <= buf->maxlen);
-	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(i));
-	buf->len += sizeof(i);
+	Assert(buf->len + sizeof(int64) <= buf->maxlen);
+	memcpy((char *pg_restrict) (buf->data + buf->len), &ni, sizeof(int64));
+	buf->len += sizeof(int64);
 }
 
 /*
@@ -131,7 +131,7 @@ pq_writestring(StringInfoData *pg_restrict buf, const char *pg_restrict str)
 static inline void
 pq_sendint8(StringInfo buf, int8 i)
 {
-	enlargeStringInfo(buf, sizeof(i));
+	enlargeStringInfo(buf, sizeof(int8));
 	pq_writeint8(buf, i);
 }
 
@@ -139,7 +139,7 @@ pq_sendint8(StringInfo buf, int8 i)
 static inline void
 pq_sendint16(StringInfo buf, int16 i)
 {
-	enlargeStringInfo(buf, sizeof(i));
+	enlargeStringInfo(buf, sizeof(int16));
 	pq_writeint16(buf, i);
 }
 
@@ -147,7 +147,7 @@ pq_sendint16(StringInfo buf, int16 i)
 static inline void
 pq_sendint32(StringInfo buf, int32 i)
 {
-	enlargeStringInfo(buf, sizeof(i));
+	enlargeStringInfo(buf, sizeof(int32));
 	pq_writeint32(buf, i);
 }
 
@@ -155,7 +155,7 @@ pq_sendint32(StringInfo buf, int32 i)
 static inline void
 pq_sendint64(StringInfo buf, int64 i)
 {
-	enlargeStringInfo(buf, sizeof(i));
+	enlargeStringInfo(buf, sizeof(int64));
 	pq_writeint64(buf, i);
 }
 
@@ -169,7 +169,7 @@ pq_sendbyte(StringInfo buf, int8 byt)
 /*
  * Append a binary integer to a StringInfo buffer
  *
- * This function is deprecated.
+ * This function is deprecated; prefer use of the functions above.
  */
 static inline void
 pq_sendint(StringInfo buf, int i, int b)