doc: git-checkout: clarify intro

- Reduce use of jargon ("index", "pathspec", "tree") by using an
  example, and by mirroring the "switch" and "restore" language from the
  first line of the man page.
- Reference and clarify the ARGUMENT DISAMBIGUATION section, as well
  as fixing a small error (in "`git checkout abc`, `abc` is a commit,
  not a `_<tree-ish>_`)

Signed-off-by: Julia Evans <[email protected]>

Author: jvns

Documentation/git-checkout.adoc

@@ -20,10 +20,12 @@ git checkout (-p|--patch) [<tree-ish>] [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
-Updates files in the working tree to match the version in the index
-or the specified tree.  If no pathspec was given, `git checkout` will
-also update `HEAD` to set the specified branch as the current
-branch.
+`git checkout` has two main modes: it can
+**switch branches**, for example with `git checkout <branch>`, and
+**restore files from a different version**, for example with
+`git checkout <commit> <filename>` or `git checkout <filename>`
+
+See ARGUMENT DISAMBIGUATION below for how Git decides which one to do.
 
 `git checkout [<branch>]`::
 	To prepare for working on _<branch>_, switch to it by updating
@@ -511,14 +513,18 @@ $ git log -g -2 HEAD
 ARGUMENT DISAMBIGUATION
 -----------------------
 
-When there is only one argument given and it is not `--` (e.g. `git
-checkout abc`), and when the argument is both a valid _<tree-ish>_
-(e.g. a branch `abc` exists) and a valid _<pathspec>_ (e.g. a file
-or a directory whose name is "abc" exists), Git would usually ask
-you to disambiguate.  Because checking out a branch is so common an
-operation, however, `git checkout abc` takes "abc" as a _<tree-ish>_
-in such a situation.  Use `git checkout -- <pathspec>` if you want
-to checkout these paths out of the index.
+When you run `git checkout <something>`, Git tries to guess whether
+`<something>` is intended to be a branch, a commit, or a set of file(s),
+and then either switches to that branch or commit or restores the
+specified files.
+
+If there's any ambiguity, Git will treat `<something>` as a branch or
+commit, but you can use the double dash `--` to force Git to treat the
+parameter as a list of files and/or directories, like this:
+
+----------
+git checkout -- file.txt
+----------
 
 EXAMPLES
 --------