* doc/environment-gdb.scm: Change to use 'modify-inputs'.
* doc/environment-gdb.scm: New file. * doc.am (EXTRA_DIST): Add it. * doc/guix.texi (Invoking guix environment): Clarify wording of the first example. Add a --ad-hoc example upfront. Add an example for '-e', and one for '-l'.