config: Rename "Asynchronous Behavior" to "I/O"

"I/O" seems more descriptive than "Asynchronous Behavior", given that some of
the GUCs in the section don't relate to anything asynchronous.

Most other abbreviations in the config sections are un-abbreviated, but
"Input/Output" seems less likely to be helpful than just IO or I/O.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/x3tlw2jk5gm3r3mv47hwrshffyw7halpczkfbk3peksxds7bvc@lguk43z3bsyq

doc/src/sgml/config.sgml

@@ -2506,8 +2506,8 @@ include_dir 'conf.d'
      </para>
     </sect2>
 
-    <sect2 id="runtime-config-resource-async-behavior">
-     <title>Asynchronous Behavior</title>
+    <sect2 id="runtime-config-resource-io">
+     <title>I/O</title>
 
      <variablelist>
       <varlistentry id="guc-backend-flush-after" xreflabel="backend_flush_after">

src/backend/utils/misc/guc_tables.c

@@ -682,7 +682,7 @@ const char *const config_group_names[] =
 	[RESOURCES_DISK] = gettext_noop("Resource Usage / Disk"),
 	[RESOURCES_KERNEL] = gettext_noop("Resource Usage / Kernel Resources"),
 	[RESOURCES_BGWRITER] = gettext_noop("Resource Usage / Background Writer"),
-	[RESOURCES_ASYNCHRONOUS] = gettext_noop("Resource Usage / Asynchronous Behavior"),
+	[RESOURCES_IO] = gettext_noop("Resource Usage / I/O"),
 	[RESOURCES_WORKER_PROCESSES] = gettext_noop("Resource Usage / Worker Processes"),
 	[WAL_SETTINGS] = gettext_noop("Write-Ahead Log / Settings"),
 	[WAL_CHECKPOINTS] = gettext_noop("Write-Ahead Log / Checkpoints"),
@@ -3181,7 +3181,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"effective_io_concurrency",
 			PGC_USERSET,
-			RESOURCES_ASYNCHRONOUS,
+			RESOURCES_IO,
 			gettext_noop("Number of simultaneous requests that can be handled efficiently by the disk subsystem."),
 			NULL,
 			GUC_EXPLAIN
@@ -3195,7 +3195,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"maintenance_io_concurrency",
 			PGC_USERSET,
-			RESOURCES_ASYNCHRONOUS,
+			RESOURCES_IO,
 			gettext_noop("A variant of \"effective_io_concurrency\" that is used for maintenance work."),
 			NULL,
 			GUC_EXPLAIN
@@ -3210,7 +3210,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"io_combine_limit",
 			PGC_USERSET,
-			RESOURCES_ASYNCHRONOUS,
+			RESOURCES_IO,
 			gettext_noop("Limit on the size of data reads and writes."),
 			NULL,
 			GUC_UNIT_BLOCKS
@@ -3222,7 +3222,7 @@ struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"backend_flush_after", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
+		{"backend_flush_after", PGC_USERSET, RESOURCES_IO,
 			gettext_noop("Number of pages after which previously performed writes are flushed to disk."),
 			NULL,
 			GUC_UNIT_BLOCKS

src/backend/utils/misc/postgresql.conf.sample

@@ -192,7 +192,7 @@
 #bgwriter_lru_multiplier = 2.0		# 0-10.0 multiplier on buffers scanned/round
 #bgwriter_flush_after = 0		# measured in pages, 0 disables
 
-# - Asynchronous Behavior -
+# - I/O -
 
 #backend_flush_after = 0		# measured in pages, 0 disables
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching

src/include/utils/guc_tables.h

@@ -64,7 +64,7 @@ enum config_group
 	RESOURCES_DISK,
 	RESOURCES_KERNEL,
 	RESOURCES_BGWRITER,
-	RESOURCES_ASYNCHRONOUS,
+	RESOURCES_IO,
 	RESOURCES_WORKER_PROCESSES,
 	WAL_SETTINGS,
 	WAL_CHECKPOINTS,