Documentation/git-push: clarify the description of defaults

We describe what gets pushed by default when the command line does
not give any <refspec> under the bullet point of <refspec>.

It is a bit unfriendly to expect users to read on <refspec> when
they are not giving any in the first place.  "What gets pushed" is
determined by taking many factors (<refspec> argument being only one
of them) into account, and is a property of the entire command, not
an individual argument.  Also we do not describe "Where the push
goes" when the command line does not say.

Give the description on "what gets pushed to where" upfront before
explaining individual arguments and options.

Also update the description of <refspec> to say what it is, what it
is used for, before explaining what shape it takes.

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

Documentation/git-push.txt

@@ -23,6 +23,17 @@ You can make interesting things happen to a repository
 every time you push into it, by setting up 'hooks' there.  See
 documentation for linkgit:git-receive-pack[1].
 
+When the command line does not specify where to push with the
+`<repository>` argument, `branch.*.remote` configuration for the
+current branch is consulted to determine where to push.  If the
+configuration is missing, it defaults to 'origin'.
+
+When the command line does not specify what to push with `<refspec>...`
+arguments or `--all`, `--mirror`, `--tags` options, the command finds
+the default `<refspec>` by consulting `remote.*.push` configuration,
+and if it is not found, honors `push.default` configuration to decide
+what to push (See gitlink:git-config[1] for the meaning of `push.default`).
+
 
 OPTIONS[[OPTIONS]]
 ------------------
@@ -33,13 +44,10 @@ OPTIONS[[OPTIONS]]
 	of a remote (see the section <<REMOTES,REMOTES>> below).
 
 <refspec>...::
+	Specify what destination ref to update with what source object.
 	The format of a <refspec> parameter is an optional plus
-	`+`, followed by the source ref <src>, followed
+	`+`, followed by the source object <src>, followed
 	by a colon `:`, followed by the destination ref <dst>.
-	It is used to specify with what <src> object the <dst> ref
-	in the remote repository is to be updated.  If not specified,
-	the behavior of the command is controlled by the `push.default`
-	configuration variable.
 +
 The <src> is often the name of the branch you would want to push, but
 it can be any arbitrary "SHA-1 expression", such as `master~4` or
@@ -65,10 +73,7 @@ the remote repository.
 The special refspec `:` (or `+:` to allow non-fast-forward updates)
 directs git to push "matching" branches: for every branch that exists on
 the local side, the remote side is updated if a branch of the same name
-already exists on the remote side.  This is the default operation mode
-if no explicit refspec is found (that is neither on the command line
-nor in any Push line of the corresponding remotes file---see below) and
-no `push.default` configuration variable is set.
+already exists on the remote side.
 
 --all::
 	Instead of naming each ref to push, specifies that all