Improve documentation of 'set-face-font'

* lisp/faces.el (set-face-font):
* doc/emacs/frames.texi (Fonts):
* doc/lispref/display.texi (Face Attributes)
(Attribute Functions): Clarify and correct the documentation
of set-face-font and related descriptions of font
specifications.  (Bug#14647)
1 job for master in 75 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #4075
test-all

01:15:17

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1