doc: clarify boundaries of 'git worktree list --porcelain'

Defined delimiters for 'git worktree list --porcelain' make the format
easier to parse in scripts. For example

	sed -n '/^worktree ID$/,/^$/p'

extracts only the information for the worktree 'ID'.

The format did not changed since [1], only the guaranty is added.

[1] bb9c03b (worktree: add 'list' command, 2015-10-08)

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: asheiduk

Documentation/git-worktree.txt

@@ -262,8 +262,8 @@ Porcelain Format
 The porcelain format has a line per attribute.  Attributes are listed with a
 label and value separated by a single space.  Boolean attributes (like 'bare'
 and 'detached') are listed as a label only, and are only present if and only
-if the value is true.  An empty line indicates the end of a worktree.  For
-example:
+if the value is true.  The first attribute of a worktree is always `worktree`,
+an empty line indicates the end of the record.  For example:
 
 ------------
 $ git worktree list --porcelain