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 <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>