Doc: #pragma pack documentation cleanup [PR114957] [PR78008] [PR60972]

Doc: #pragma pack documentation cleanup [PR114957] [PR78008] [PR60972]

This patch addresses a number of issues with the documentation of

- None of the things in this section had @cindex entries [PR114957].

- The document formatting didn't match that of other #pragma
documentation sections.

- The effect of #pragma pack(0) wasn't documented [PR78008].

- There's a long-standing bug [PR60972] reporting that #pragma pack
and the __attribute__(packed) don't get along well.  It seems worthwhile
to warn users about that since elsewhere pragmas are cross-referenced
with related or equivalent attributes.

gcc/ChangeLog
	PR c/114957
	PR c/78008
	PR c++/60972
	* doc/extend.texi (Structure-Layout Pragmas):  Add @cindex
	entries	and reformat the pragma descriptions to match the markup
	used for other pragmas.  Document what #pragma pack(0) does.
	Add cross-references to similar attributes.