More detailed font documentation (gravitystorm#3735)

INSTALL.md

@@ -66,7 +66,11 @@ DejaVu Sans is used as an optional fallback font for systems without Noto Sans.
 
 Hanazono is used a fallback for seldom used CJK characters that are not covered by Noto.
 
-Unifont is used as a last resort fallback, with it's excellent coverage, common presence on machines, and ugly look.
+Unifont is used as a last resort fallback, with it's excellent coverage, common presence on machines, and ugly look. For compatibility reasons, we support two Linux-distributions-specific versions of Unifont, therefor it's expected that you *always* get a warning about a missing Unifont version.
+
+If you do not install all the fonts, the rendering itself will not break, but missing glyphs will be ugly.
+
+For more details, see the documentation at fonts.mss.
 
 ### Installation on Ubuntu/Debian
 
@@ -76,7 +80,7 @@ On Ubuntu 16.04 or Debian Testing you can download and install most of the requi
 sudo apt-get install fonts-noto-cjk fonts-noto-hinted fonts-noto-unhinted fonts-hanazono ttf-unifont
 ```
 
-Noto Emoji Regular can be downloaded [from the Noto Emoji repository](https://github.com/googlei18n/noto-emoji).
+Noto Emoji Regular (*not* Noto Color Emoji) can be downloaded [from the Noto Emoji repository](https://github.com/googlei18n/noto-emoji).
 
 It might be useful to have a more recent version of the fonts for [rare non-latin scripts](#non-latin-scripts). The current upstream font release has also some more scripts and style variants than in the Ubuntu package. It can be installed [from source](https://github.com/googlei18n/noto-fonts/blob/master/FAQ.md#where-are-the-fonts).
 

fonts.mss

@@ -55,7 +55,14 @@ without Noto Sans Arabic UI.
 6. Fallback fonts. Hanazono covers almost all CJK characters, even in Unicode
 Plane 2. Unifont is a fallback of last resort with full coverage in Plane 0
 (Unifont Medium), some coverage in Plane 1 (Unifont Upper Medium) and no
-coverage in Plane 2. Unifont Medium Sample would cover the BMP PUA with
+coverage in Plane 2. Unifont has different font names on different Linux
+distributions (and sometimes even in different versions of the same Linux
+distribution), see #429 and #2924 for details. We have therefore both,
+“Unifont Medium” and “unifont Medium” in our list, so on almost all
+distributions we catch the font if installed – and you will always get an error
+message for the other missing font name. We prefer having an error message than
+requiring our users to customize the font list depending on their Linux
+distribution. Unifont Medium Sample would cover the BMP PUA with
 replacement characters, but cannot be used because Mapnik does not
 support SBIT TTF.
 */