glossary: describe "worktree"

We have description on "per worktree ref", but "worktree" is not
described in the glossary.  We do have "working tree", though.

Casually put, a "working tree" is what your editor and compiler
interacts with.  "worktree" is a mechanism to allow one or more
"working tree"s to be attached to a repository and used to check out
different commits and branches independently, which includes not
just a "working tree" but also repository metadata like HEAD, the
index to support simultaneous use of them.  Historically, we used
these terms interchangeably but we have been trying to use "working
tree" when we mean it, instead of "worktree".

Most of the existing references to "working tree" in the glossary do
refer primarily to the working tree portion, except for one that
said refs like HEAD and refs/bisect/* are per "working tree", but it
is more precise to say they are per "worktree".

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

Documentation/glossary-content.txt

@@ -312,7 +312,7 @@ Pathspecs are used on the command line of "git ls-files", "git
 ls-tree", "git add", "git grep", "git diff", "git checkout",
 and many other commands to
 limit the scope of operations to some subset of the tree or
-worktree.  See the documentation of each command for whether
+working tree.  See the documentation of each command for whether
 paths are relative to the current directory or toplevel.  The
 pathspec syntax is as follows:
 +
@@ -446,7 +446,7 @@ exclude;;
 	interface than the <<def_plumbing,plumbing>>.
 
 [[def_per_worktree_ref]]per-worktree ref::
-	Refs that are per-<<def_working_tree,worktree>>, rather than
+	Refs that are per-<<def_worktree,worktree>>, rather than
 	global.  This is presently only <<def_HEAD,HEAD>> and any refs
 	that start with `refs/bisect/`, but might later include other
 	unusual refs.
@@ -669,3 +669,12 @@ The most notable example is `HEAD`.
 	The tree of actual checked out files.  The working tree normally
 	contains the contents of the <<def_HEAD,HEAD>> commit's tree,
 	plus any local changes that you have made but not yet committed.
+
+[[def_worktree]]worktree::
+	A repository can have zero (i.e. bare repository) or one or
+	more worktrees attached to it. One "worktree" consists of a
+	"working tree" and repository metadata, most of which are
+	shared among other worktrees of a single repository, and
+	some of which are maintained separately per worktree
+	(e.g. the index, HEAD and pseudorefs like MERGE_HEAD,
+	per-worktree refs and per-worktree configuration file).