Update docs on behaviour of 'core.sharedRepository' and 'git init --shared'

This documentation update is needed to reflect the recent changes where
"core.sharedRepository = 0mode" was changed to set, not loosen, the
repository permissions.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jherland

Documentation/config.txt

@@ -290,8 +290,10 @@ core.sharedRepository::
 	group-shareable. When 'umask' (or 'false'), git will use permissions
 	reported by umask(2). When '0xxx', where '0xxx' is an octal number,
 	files in the repository will have this mode value. '0xxx' will override
-	user's umask value, and thus, users with a safe umask (0077) can use
-	this option. Examples: '0660' is equivalent to 'group'. '0640' is a
+	user's umask value (whereas the other options will only override
+	requested parts of the user's umask value). Examples: '0660' will make
+	the repo read/write-able for the owner and group, but inaccessible to
+	others (equivalent to 'group' unless umask is e.g. '0022'). '0640' is a
 	repository that is group-readable but not group-writable.
 	See linkgit:git-init[1]. False by default.
 

Documentation/git-init.txt

@@ -54,15 +54,21 @@ is given:
 
  - 'group' (or 'true'): Make the repository group-writable, (and g+sx, since
    the git group may be not the primary group of all users).
+   This is used to loosen the permissions of an otherwise safe umask(2) value.
+   Note that the umask still applies to the other permission bits (e.g. if
+   umask is '0022', using 'group' will not remove read privileges from other
+   (non-group) users). See '0xxx' for how to exactly specify the repository
+   permissions.
 
  - 'all' (or 'world' or 'everybody'): Same as 'group', but make the repository
    readable by all users.
 
- - '0xxx': '0xxx' is an octal number and each file will have mode '0xxx'
-   Any option except 'umask' can be set using this option. '0xxx' will
-   override users umask(2) value, and thus, users with a safe umask (0077)
-   can use this option. '0640' will create a repository which is group-readable
-   but not writable. '0660' is equivalent to 'group'.
+ - '0xxx': '0xxx' is an octal number and each file will have mode '0xxx'.
+   '0xxx' will override users' umask(2) value (and not only loosen permissions
+   as 'group' and 'all' does). '0640' will create a repository which is
+   group-readable, but not group-writable or accessible to others. '0660' will
+   create a repo that is readable and writable to the current user and group,
+   but inaccessible to others.
 
 By default, the configuration flag receive.denyNonFastForwards is enabled
 in shared repositories, so that you cannot force a non fast-forwarding push