shell doc: emphasize purpose and security model
shell doc: emphasize purpose and security model

The original git-shell(1) manpage emphasized that the shell supports
only git transport commands.  As the shell gained features, that
emphasis and focus in the manual has been lost.  Bring it back by
splitting the manpage into a few short sections and fleshing out each:

 - SYNOPSIS, describing how the shell gets used in practice
 - DESCRIPTION, which gives an overview of the purpose and guarantees
   provided by this restricted shell
 - COMMANDS, listing supported commands and restrictions on the
   arguments they accept
 - INTERACTIVE USE, describing the interactive mode

Also add a "see also" section with related reading.

Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git