doc: git-checkout: clarify restoring files section

From user feedback on this section: 3 users don't know what "tree-ish"
means and 3 users don't know what "pathspec" means. One user also says
that the section is very confusing and that they don't understand what
the "index" is.

From conversations on Mastodon, several users said that their impression
is that "the index" means the same thing as "HEAD". It would be good to
give users a hint that `git checkout file.txt` is not always the same as
`git checkout HEAD file.txt`: it's different if there are staged
changes to `file.txt`.

Replace "pathspec" in the description with "the specified files and/or
directories", so that hopefully users can infer that "pathspec" is
a way to specify files/directories, say "commit or tree" instead of
"tree-ish", and add an example to give a common use case for
`git checkout file.txt` (discarding unstaged changes) to hopefully make
the section more accessible to users who do not know what the term
"index" means.

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

Author: jvns

Documentation/git-checkout.adoc

@@ -73,23 +73,29 @@ Omitting _<branch>_ detaches `HEAD` at the tip of the current branch.
 `git checkout <tree-ish> [--] <pathspec>...`::
 `git checkout <tree-ish> --pathspec-from-file=<file> [--pathspec-file-nul]`::
 
-	Overwrite both the index and the working tree with the
-	contents at the _<tree-ish>_ for the files that match the pathspec.
+	Replace the specified files and/or directories with the version from
+	the given commit or tree and add them to the index
+	(also known as "staging area").
++
+For example, `git checkout main file.txt` will replace `file.txt`
+with the version from `main`.
 
 `git checkout [-f|--ours|--theirs|-m|--conflict=<style>] [--] <pathspec>...`::
 `git checkout [-f|--ours|--theirs|-m|--conflict=<style>] --pathspec-from-file=<file> [--pathspec-file-nul]`::
 
-	Overwrite working tree with the contents in the index for the files
-	that match the pathspec.
+	Replace the specified files and/or directories with the version from
+	the index.
 +
-The index may contain unmerged entries because of a previous failed merge.
-By default, if you try to check out such an entry from the index, the
-checkout operation will fail and nothing will be checked out.
-Using `-f` will ignore these unmerged entries.  The contents from a
-specific side of the merge can be checked out of the index by
-using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
-file can be discarded to re-create the original conflicted merge result.
-
+For example, if you check out a commit, edit `file.txt`, and then
+decide those changes were a mistake, `git checkout file.txt` will
+discard any unstaged changes to `file.txt`.
++
+This will fail if the file has a merge conflict and you haven't yet run
+`git add file.txt` (or something equivalent) to mark it as resolved.
+You can use `-f` to ignore the unmerged files instead of failing, use
+`--ours` or `--theirs` to replace them with the version from a specific
+side of the merge, or use `-m` to replace them with the original
+conflicted merge result.
 `git checkout (-p|--patch) [<tree-ish>] [--] [<pathspec>...]`::
 	This is similar to the previous two modes, but lets you use the
 	interactive interface to show the "diff" output and choose which