Document WESTON_VERSION_AT_LEAST() macro behavior

Document WESTON_VERSION_AT_LEAST() macro behavior

Adding this comment to explain the behavior:

  This macro may not do what you expect.  Weston doesn't guarantee any
  stable API between 1.X and 1.Y, and thus this macro will return
  FALSE on any WESTON_VERSION_AT_LEAST(1,X,0) if the actualy version
  is 1.Y.0 and X !=Y).  In particular, it fail if X < Y, that is,
  1.3.0 is considered to not be "at least" 1.4.0.

  If you want to test for the version number being 1.3.0 or above or
  maybe in a range (eg 1.2.0 to 1.4.0), just use the WESTON_VERSION_*
  defines above directly.

Version number testing is the one thing we can't break in the weston API,
so we'll have to settle for documenting the behavior and recommending
using the version number macros directly.

https://bugs.freedesktop.org/show_bug.cgi?id=74023