Document the double-dash "rev -- path" disambiguator

This is a very well established command line convention that old residents
of the git mailing list knew by heart and nobody even thought about
documenting it explicitly, which was not very nice.

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

Documentation/gitcli.txt

@@ -13,8 +13,37 @@ gitcli
 DESCRIPTION
 -----------
 
-This manual describes best practice in how to use git CLI.  Here are
-the rules that you should follow when you are scripting git:
+This manual describes the convention used throughout git CLI.
+
+Many commands take revisions (most often "commits", but sometimes
+"tree-ish", depending on the context and command) and paths as their
+arguments.  Here are the rules:
+
+ * Revisions come first and then paths.
+   E.g. in `git diff v1.0 v2.0 arch/x86 include/asm-x86`,
+   `v1.0` and `v2.0` are revisions and `arch/x86` and `include/asm-x86`
+   are paths.
+
+ * When an argument can be misunderstood as either a revision or a path,
+   they can be disambiguated by placing `\--` between them.
+   E.g. `git diff \-- HEAD` is, "I have a file called HEAD in my work
+   tree.  Please show changes between the version I staged in the index
+   and what I have in the work tree for that file". not "show difference
+   between the HEAD commit and the work tree as a whole".  You can say
+   `git diff HEAD \--` to ask for the latter.
+
+ * Without disambiguating `\--`, git makes a reasonable guess, but errors
+   out and asking you to disambiguate when ambiguous.  E.g. if you have a
+   file called HEAD in your work tree, `git diff HEAD` is ambiguous, and
+   you have to say either `git diff HEAD \--` or `git diff \-- HEAD` to
+   disambiguate.
+
+When writing a script that is expected to handle random user-input, it is
+a good practice to make it explicit which arguments are which by placing
+disambiguating `\--` at appropriate places.
+
+Here are the rules regarding the "flags" that you should follow when you are
+scripting git:
 
  * it's preferred to use the non dashed form of git commands, which means that
    you should prefer `"git foo"` to `"git-foo"`.
@@ -34,8 +63,8 @@ the rules that you should follow when you are scripting git:
    if you happen to have a file called `HEAD` in the work tree.
 
 
-ENHANCED CLI
-------------
+ENHANCED OPTION PARSER
+----------------------
 From the git 1.5.4 series and further, many git commands (not all of them at the
 time of the writing though) come with an enhanced option parser.