Improve explanation of available font backends under X

* frames.texi (Font and Color Parameters): Clarify that you can't
have HarfBuzz and non-HarfBuzz at the same time for xft and cairo
font backends.

Author: rpluim

doc/lispref/frames.texi

@@ -2294,14 +2294,17 @@ variable do not take effect immediately, only when you specify the
 @item font-backend
 A list of symbols, specifying the @dfn{font backends} to use for
 drawing characters on the frame, in order of priority.  In Emacs built
-without Cairo drawing on X, there are currently three available font
-backends: @code{x} (the X core font driver), @code{xft} (the Xft font
-driver), and @code{xfthb} (the Xft font driver with HarfBuzz text
-shaping).  If built with the Cairo drawing, there are also three
-available font backends on X: @code{x}, @code{ftcr} (the FreeType font
-driver on Cairo), and @code{ftcrhb} (the FreeType font driver on Cairo
-with HarfBuzz text shaping).  On MS-Windows, there are currently three
-available font backends: @code{gdi} (the core MS-Windows font driver),
+without Cairo drawing on X, there are currently three potentially
+available font backends: @code{x} (the X core font driver), @code{xft}
+(the Xft font driver), and @code{xfthb} (the Xft font driver with
+HarfBuzz text shaping).  If built with Cairo drawing, there are also
+three potentially available font backends on X: @code{x}, @code{ftcr}
+(the FreeType font driver on Cairo), and @code{ftcrhb} (the FreeType
+font driver on Cairo with HarfBuzz text shaping).  Note that the
+@code{ftcr} and @code{ftcrhb} drivers are mutually exclusive (and
+similarly for @code{xft} and @code{xfthb}), with the choice being made
+at build time.  On MS-Windows, there are currently three available
+font backends: @code{gdi} (the core MS-Windows font driver),
 @code{uniscribe} (font driver for OTF and TTF fonts with text shaping
 by the Uniscribe engine), and @code{harfbuzz} (font driver for OTF and
 TTF fonts with HarfBuzz text shaping) (@pxref{Windows Fonts,,, emacs,