man: document font syntaxes

Change-Id: Id441d4df192c47388af6b5da306f14d90f066d18
Reviewed-on: https://plan9port-review.googlesource.com/1173
Reviewed-by: Russ Cox <rsc@swtch.com>

man/man1/9term.1

@@ -57,6 +57,9 @@ is not given,
 uses the imported value of
 .B $font
 if set; otherwise it uses the graphics system default.
+(See
+.IR font (7)
+for a full discussion of font syntaxes.)
 .PP
 .I 9term
 runs the given command in the window, or 

man/man1/mc.1

@@ -44,3 +44,14 @@ is printed separately.
 .I lc
 in
 .IR ls (1)
+.SH BUGS
+On systems with high-DPI screens,
+.I 9term
+and
+.I acme
+do not have one current font.
+They have a pair of current fonts, one font used when the window
+on a low-DPI screen and one used when the window is on a 
+high-DPI screen.
+.I Mc
+always uses the low-DPI font for columnation decisions.