Fix description of I/O timing info for shared buffers in EXPLAIN (BUFFERS)

This fixes an error introduced by efb0ef909f60, that changed the
description of this field to "shared/local" while these I/O timings
relate to shared buffers.  This information is available when
track_io_timing is enabled.  Note that HEAD has added new counters for
local buffers in 295c36c0c1fa, so there is no need to touch it.  The
description is updated to "shared" to be compatible with HEAD.

Per discussion with Nazir Bilal Yavuz and Hubert Depesz Lubaczewski,
whose EXPLAIN analyzer tool was not actually able to parse the previous
term because of the slash character.

Discussion: https://postgr.es/m/ZTCTiUqm_H3iBihl@paquier.xyz
Backpatch-through: 15

meson.build

@@ -1131,7 +1131,7 @@ if not get_option('readline').disabled()
 
     if not at_least_one_header_found
       error('''readline header not found
-If you have @0@ already installed, see meson-log/meson-log.txt for details on the
+If you have @0@ already installed, see meson-logs/meson-log.txt for details on the
 failure. It is possible the compiler isn't looking in the proper directory.
 Use -Dreadline=disabled to disable readline support.'''.format(readline_dep))
     endif

src/backend/commands/explain.c

@@ -3632,7 +3632,7 @@ show_buffer_usage(ExplainState *es, const BufferUsage *usage, bool planning)
 
 			if (has_timing)
 			{
-				appendStringInfoString(es->str, " shared/local");
+				appendStringInfoString(es->str, " shared");
 				if (!INSTR_TIME_IS_ZERO(usage->blk_read_time))
 					appendStringInfo(es->str, " read=%0.3f",
 									 INSTR_TIME_GET_MILLISEC(usage->blk_read_time));