doc: Clarify __builtin_return_address [PR94891]

doc: Clarify __builtin_return_address [PR94891]

The expected semantics and valid usage of __builtin_return_address is
not clear since it exposes implementation internals that are normally
not meaningful to portable c code.

This documentation change tries to clarify the semantics in case the
return address is stored in a mangled form. This affects AArch64 when
pointer authentication is used for the return address signing (i.e.
-mbranch-protection=pac-ret).

2020-07-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

gcc/ChangeLog:

	PR target/94891
	* doc/extend.texi: Update the text for  __builtin_return_address.

(cherry picked from commit 6a391e06f953c3390b14020d8cacb6d55f81b2b9)