[DOC] shell startup files, clarifications and fixes.

[DOC] shell startup files, clarifications and fixes.

When Bash is started non-interactively, it runs the script pointed by
the BASH_ENV environment variable, not .bashrc.  While at it, mention
Z shell in the warning too, and mention non-interactive mode
explicitly.

gdb/doc/
2013-11-06  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Starting) <set/show startup-with-shell>: Mention
	non-interactive mode.
	(Environment) <shell startup files warning>: Mention
	non-interactive mode.  Mention .zshenv for Z shell, and talk about
	BASH_ENV instead of .bashrc for BASH.