Fix documentation of FmgrInfo.fn_nargs.

Some ancient comments claimed that fn_nargs could be -1 to indicate a
variable number of input arguments; but this was never implemented, and
is at variance with what we ultimately did with "variadic" functions.
Update the comments.

src/backend/utils/fmgr/README

@@ -67,7 +67,7 @@ typedef struct
 {
     PGFunction  fn_addr;    /* pointer to function or handler to be called */
     Oid         fn_oid;     /* OID of function (NOT of handler, if any) */
-    short       fn_nargs;   /* 0..FUNC_MAX_ARGS, or -1 if variable arg count */
+    short       fn_nargs;   /* number of input args (0..FUNC_MAX_ARGS) */
     bool        fn_strict;  /* function is "strict" (NULL in => NULL out) */
     bool        fn_retset;  /* function returns a set (over multiple calls) */
     unsigned char fn_stats; /* collect stats if track_functions > this */

src/include/fmgr.h

@@ -54,8 +54,7 @@ typedef struct FmgrInfo
 {
 	PGFunction	fn_addr;		/* pointer to function or handler to be called */
 	Oid			fn_oid;			/* OID of function (NOT of handler, if any) */
-	short		fn_nargs;		/* 0..FUNC_MAX_ARGS, or -1 if variable arg
-								 * count */
+	short		fn_nargs;		/* number of input args (0..FUNC_MAX_ARGS) */
 	bool		fn_strict;		/* function is "strict" (NULL in => NULL out) */
 	bool		fn_retset;		/* function returns a set */
 	unsigned char fn_stats;		/* collect stats if track_functions > this */