[doc, committed] make #pragma once documentation easier to find

[Sandra Loosemore]

This patch is for PR 47823, which was about the documentation for 
"#pragma once" being buried in a strange location in the CPP manual. 
I've put the primary documentation for it with the other preprocessor 
pragmas in the CPP manual, and added a blurb to the pragmas chapter in 
the GCC manual explaining that preprocessor pragmas are documented in 
the CPP manual.

-Sandra
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