Documentation/git-worktree: add high-level 'lock' overview

Due to the (current) absence of a "git worktree lock" command, locking
a worktree's administrative files to prevent automatic pruning is a
manual task, necessarily requiring low-level understanding of linked
worktree functionality. However, this level of detail does not belong
in the high-level DESCRIPTION section, so add a generalized discussion
of locking to DESCRIPTION and move the technical information to DETAILS.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -37,15 +37,11 @@ at least one git command inside the linked working directory
 (e.g. `git status`) in order to update its administrative files in the
 repository so that they do not get automatically pruned.
 
-To prevent a $GIT_DIR/worktrees entry from from being pruned (which
-can be useful in some situations, such as when the
-entry's working tree is stored on a portable device), add a file named
-'locked' to the entry's directory. The file contains the reason in
-plain text. For example, if a linked working tree's `.git` file points
-to `/path/main/.git/worktrees/test-next` then a file named
-`/path/main/.git/worktrees/test-next/locked` will prevent the
-`test-next` entry from being pruned.  See
-linkgit:gitrepository-layout[5] for details.
+If a linked working tree is stored on a portable device or network share
+which is not always mounted, you can prevent its administrative files from
+being pruned by creating a file named 'lock' alongside the other
+administrative files, optionally containing a plain text reason that
+pruning should be suppressed. See section "DETAILS" for more information.
 
 COMMANDS
 --------
@@ -99,6 +95,16 @@ thumb is do not make any assumption about whether a path belongs to
 $GIT_DIR or $GIT_COMMON_DIR when you need to directly access something
 inside $GIT_DIR. Use `git rev-parse --git-path` to get the final path.
 
+To prevent a $GIT_DIR/worktrees entry from from being pruned (which
+can be useful in some situations, such as when the
+entry's working tree is stored on a portable device), add a file named
+'locked' to the entry's directory. The file contains the reason in
+plain text. For example, if a linked working tree's `.git` file points
+to `/path/main/.git/worktrees/test-next` then a file named
+`/path/main/.git/worktrees/test-next/locked` will prevent the
+`test-next` entry from being pruned.  See
+linkgit:gitrepository-layout[5] for details.
+
 BUGS
 ----
 Multiple checkout support for submodules is incomplete. It is NOT