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Add a note to PG_TRY's documentation about volatile safety.

We had better memorialize what the actual requirements are for this.

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Original file line number	Diff line number	Diff line change
`@@ -332,6 +332,13 @@ extern PGDLLIMPORT ErrorContextCallback *error_context_stack;`
`332`	`332`	`* not without taking thought for what will happen during ereport(FATAL).`
`333`	`333`	`* The PG_ENSURE_ERROR_CLEANUP macros provided by storage/ipc.h may be`
`334`	`334`	`* helpful in such cases.`
	`335`	`+ *`
	`336`	`+ * Note: if a local variable of the function containing PG_TRY is modified`
	`337`	`+ * in the PG_TRY section and used in the PG_CATCH section, that variable`
	`338`	`+ * must be declared "volatile" for POSIX compliance. This is not mere`
	`339`	`+ * pedantry; we have seen bugs from compilers improperly optimizing code`
	`340`	`+ * away when such a variable was not marked. Beware that gcc's -Wclobbered`
	`341`	`+ * warnings are just about entirely useless for catching such oversights.`
`335`	`342`	`*----------`
`336`	`343`	`*/`
`337`	`344`	`#definePG_TRY() \`

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