Fine tune exec-file-mismatch help and documentation.

Fine tune exec-file-mismatch help and documentation.

It was deemed better to explicitly mention in help and doc that build IDs
are used for comparison, and that symbols are loaded when asking to
load the exec-file.

This is V2, fixing 2 typos and replacing 'If the user asks to load'
by 'If the user confirms loading', as suggested by Pedro.

gdb/ChangeLog
2020-07-11  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* exec.c (_initialize_exec): Update exec-file-mismatch help.

gdb/doc/ChangeLog
2020-07-11  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.texinfo (Attach): Update exec-file-mismatch doc.