Document 'set|show exec-file-mismatch (ask|warn|off)'

Document 'set|show exec-file-mismatch (ask|warn|off)'

Mention in NEWS the new option and the set/show commands.

Document in gdb.texinfo the new option and the set/show commands.

gdb/ChangeLog
2020-01-25  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* NEWS: Mention the new option and the set/show commands.

gdb/doc/ChangeLog
2020-01-25  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.texinfo (Attach): Document the new option and the
	set/show commands.
	(Connecting): Reference the exec-file-mismatch option.