Revert "Disable region wrapping for trigger key expansion"

This reverts commit f8d1af1f50f42ac1689c583c152c3b15e8774ae4.

Disabling this for trigger key introduced an inconsistency that is more
complicated to implement and document than simply fixing region wrapping
for trigger key expanded snippets.

doc/snippet-expansion.org

@@ -29,7 +29,7 @@
 
    - Expanding from emacs-lisp code
 
-** <<Trigger key>>
+** Trigger key
 
 [[sym:yas-expand][=yas-expand=]] tries to expand a /snippet abbrev/ (also known as
 /snippet key/) before point.
@@ -91,10 +91,6 @@ the region contents into the =$0= placeholder of a snippet expanded by
 [[#yas-insert-snippet][=yas-insert-snippet=]].  Setting it to a character value (e.g. =?0=)
 will insert the contents of corresponding register.
 
-Note a setting of t is disabled when expanding via [[trigger key]] since
-it's too awkward to highlight the relevant text while maneuvering
-point in front of the trigger key.
-
 Older (versions 0.9.1 and below) of Yasnippet, supported a setting of
 =cua= that is equivalent to =?0= but only worked with =cua-mode=
 turned on.  This setting is still supported for backwards