awful.wibox.stretch: Spell out full replacement

The function that is documented as awful.wibox.stretch is deprecated,
because it was removed (that's not a deprecation, is it?!?). For the
replacement, we used "@see stretch". However, LDoc was randomly
resolving this reference to awful.wibar.stretch (good) or
awful.wibox.stretch (bad; the see points to the element where it
appears).

Fix this by spelling out the "full name" of the function in the @see.

Related-to: https://github.com/awesomeWM/awesome/issues/834
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/wibar.lua

@@ -262,7 +262,7 @@ end
 --
 -- @deprecated awful.wibox.stretch
 -- @see awful.placement
--- @see stretch
+-- @see awful.wibar.stretch
 
 --- Create a new wibox and attach it to a screen edge.
 -- You can add also position key with value top, bottom, left or right.