Documentation: explain optional arguments better

Improve the documentation of commands taking optional arguments in two
ways:

* Documents the behavior of '-O' (for grep) and '-S' (for commands
  creating commits) when used without the optional argument.

* Document the syntax of these options.

For the second point, the behavior is documented in gitcli(7), but it is
easy for users to miss, and hard for the same user to understand why e.g.
"git status -u no" does not work.

Document this explicitly in the documentation of each short option having
an optional argument: they are the most error prone since there is no '='
sign between the option and its argument.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -138,7 +138,9 @@ default.   You can use `--no-utf8` to override this.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commits.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 --continue::
 -r::

Documentation/git-cherry-pick.txt

@@ -103,7 +103,9 @@ effect to your index in a row.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commits.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 --ff::
 	If the current HEAD is the same as the parent of the

Documentation/git-commit-tree.txt

@@ -56,7 +56,9 @@ OPTIONS
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commit.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 --no-gpg-sign::
 	Countermand `commit.gpgSign` configuration variable that is

Documentation/git-commit.txt

@@ -314,7 +314,9 @@ changes to tracked files.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commit.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 --no-gpg-sign::
 	Countermand `commit.gpgSign` configuration variable that is

Documentation/git-grep.txt

@@ -165,7 +165,10 @@ OPTIONS
 	Open the matching files in the pager (not the output of 'grep').
 	If the pager happens to be "less" or "vi", and the user
 	specified only one pattern, the first file is positioned at
-	the first match automatically.
+	the first match automatically. The `pager` argument is
+	optional; if specified, it must be stuck to the option
+	without a space. If `pager` is unspecified, the default pager
+	will be used (see `core.pager` in linkgit:git-config[1]).
 
 -z::
 --null::

Documentation/git-merge.txt

@@ -67,7 +67,9 @@ include::merge-options.txt[]
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign the resulting merge commit.
+	GPG-sign the resulting merge commit. The `keyid` argument is
+	optional and defaults to the committer identity; if specified,
+	it must be stuck to the option without a space.
 
 -m <msg>::
 	Set the commit message to be used for the merge commit (in

Documentation/git-rebase.txt

@@ -285,7 +285,9 @@ which makes little sense.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commits.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 -q::
 --quiet::

Documentation/git-revert.txt

@@ -82,7 +82,9 @@ effect to your index in a row.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-	GPG-sign commits.
+	GPG-sign commits. The `keyid` argument is optional and
+	defaults to the committer identity; if specified, it must be
+	stuck to the option without a space.
 
 -s::
 --signoff::

Documentation/git-status.txt

@@ -53,8 +53,9 @@ OPTIONS
 --untracked-files[=<mode>]::
 	Show untracked files.
 +
-The mode parameter is optional (defaults to 'all'), and is used to
-specify the handling of untracked files.
+The mode parameter is used to specify the handling of untracked files.
+It is optional: it defaults to 'all', and if specified, it must be
+stuck to the option (e.g. `-uno`, but not `-u no`).
 +
 The possible options are:
 +