fetch doc: remove notes on outdated "mixed layout"

In old days before Git 1.5, it was customery for "git fetch" to use
the same local branch namespace to keep track of the remote-tracking
branches, and it was necessary to tell users not to check them out
and commit on them.  Since everybody uses the separate remote layout
these days, there is no need to warn against the practice to check
out the right-hand side of <refspec> and build on it---the RHS is
typically not even a local branch.

Incidentally, this also kills one mention of "Pull:" line of
$GIT_DIR/remotes/* configuration, which is a lot less familiar to
new people than the more modern remote.*.fetch configuration
variable.

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

Documentation/pull-fetch-param.txt

@@ -36,19 +36,6 @@ in a repository with this behavior; the pulling user simply
 must know this is the expected usage pattern for a branch.
 +
 [NOTE]
-You never do your own development on branches that appear
-on the right hand side of a <refspec> colon on `Pull:` lines;
-they are to be updated by 'git fetch'.  If you intend to do
-development derived from a remote branch `B`, have a `Pull:`
-line to track it (i.e. `Pull: B:remote-B`), and have a separate
-branch `my-B` to do your development on top of it.  The latter
-is created by `git branch my-B remote-B` (or its equivalent `git
-checkout -b my-B remote-B`).  Run `git fetch` to keep track of
-the progress of the remote side, and when you see something new
-on the remote branch, merge it into your development branch with
-`git pull . remote-B`, while you are on `my-B` branch.
-+
-[NOTE]
 There is a difference between listing multiple <refspec>
 directly on 'git pull' command line and having multiple
 `Pull:` <refspec> lines for a <repository> and running