move magit-grep into section "Acting (3)"

Also rename toplevel section "Extensions" to "Magit Extensions"
and move it into section "Miscellaneous".
sanitize interactive magit-grep
make "Submoduling" a subsection of "Acting (3)"
move magit-interactive-rebase into "Rebase" section
move magit-interactive-resolve into subsection of "Acting (3)"
make "External Tools" a subsection of "Miscellaneous"
fix typo in section heading
Merge branch 'sections'
add magit-with-git-editor-setup

The respective functionality was previously part of
`magit-interactive-rebase'.  Currently this is still the only
function that makes use of this new macro - but that will change.

Add a doc-string to make the macro less confusing.  (That's another
reason why this macro is a good idea; it gives us a place to document
the behaviour.)
improve magit-with-git-editor-setup

In the "non-default name on windows" case use default $GIT_EDITOR
instead of unsetting it.  (If the user bothered to set it she
probably prefers it over the default.)

Use dynamically bound `process-environment' instead of doing an
`unwind-protect' dance.
Merge branch 'git-editor'
fix typos in documentation