Improve documentation of 'font-spec'

* doc/lispref/display.texi (Low-Level Font):
* src/font.c (Ffont_spec): Document 'font-spec' keys that are
supported, but were undocumented.

doc/lispref/display.texi

@@ -3734,6 +3734,20 @@ Additional typographic style information for the font, such as
 The charset registry and encoding of the font, such as
 @samp{iso8859-1}.  The value should be a string or a symbol.
 
+@item :dpi
+The resolution in dots per inch for which the font is designed.  The
+value must be a non-negative number.
+
+@item :spacing
+The spacing of the font: proportional, dual, mono, or charcell.  The
+value should be either an integer (0 for proportional, 90 for dual,
+100 for mono, 110 for charcell) or a one-letter symbol (one of
+@code{P}, @code{D}, @code{M}, or @code{C}).
+
+@item :avgwidth
+The average width of the font in 1/10 pixel units.  The value should
+be a non-negative number.
+
 @item :script
 The script that the font must support (a symbol).
 

src/font.c

@@ -3934,6 +3934,23 @@ VALUE must be a non-negative integer or a floating point number
 specifying the font size.  It specifies the font size in pixels (if
 VALUE is an integer), or in points (if VALUE is a float).
 
+`:dpi'
+
+VALUE must be a non-negative number that specifies the resolution
+(dot per inch) for which the font is designed.
+
+`:spacing'
+
+VALUE specifies the spacing of the font: mono, proportional, charcell,
+or dual.  It can be either a number (0 for proportional, 90 for dual,
+100 for mono, 110 for charcell) or a 1-letter symbol: `P', `D', `M',
+or `C' (lower-case variants are also accepted).
+
+`:avgwidth'
+
+VALUE must be a non-negative integer specifying the average width of
+the font in 1/10 pixel units.
+
 `:name'
 
 VALUE must be a string of XLFD-style or fontconfig-style font name.