Document higan's video sync/audio sync issues.

docs/faq.md

@@ -1,14 +1,92 @@
+I see "tearing" when a game scrolls. How can I enable vsync?
+------------------------------------------------------------
+
+higan supports synchronizing video output
+to the display's vertical-synchronization (or "vsync") signal,
+but the option is hidden
+because it often causes more problems than it solves
+(see the next question).
+
+To enable video synchronization:
+
+  - Open the higan's configuration file, `settings.bml`
+      - On Windows, look in `%LOCALAPPDATA%\higan`
+        or beside `higan.exe`
+      - On Linux, look in `~/.local/share/higan`
+  - Open it in your favourite text editor
+    (Windows Notepad will corrupt the file,
+    use WordPad if you don't have anything better)
+  - Find the line that says "Video"
+  - Somewhere below that, find an indented line
+    that says "Synchronize:false".
+  - Change "false" to "true"
+  - Save your changes to `settings.bml`
+    and restart higan
+
+Why is video synchronization a problem for higan?
+-------------------------------------------------
+
+**The short version:**
+Turning on video synchronization
+cleans up video tearing,
+turning on audio synchronization
+cleans up audio glitches,
+but turning on both
+makes audio glitches worse.
+
+**The long version:**
+Enabling video synchronization
+locks the frame-rate of the emulated console
+to the frame-rate of your computer's display.
+If your display's refresh rate exactly matches
+the emulated console's,
+games play at the correct speed
+and everything's fine.
+
+However,
+modern 60Hz displays do not always match
+the emulated console's refresh rate:
+
+  - The Super Famicom usually runs a little faster than 60Hz
+  - the PAL variants of most consoles run at 50Hz
+  - the WonderSwan runs at 75Hz
+  - While the Game Boy does run its LCD at 60Hz
+    it can turn it off and on at any time,
+    requiring emulation to pause
+    until it can get back in sync
+    with the computer display.
+
+Because of these frame-rate differences,
+enabling video synchronization
+can force games to run
+faster or slower than intended.
+
+The consoles that higan emulates
+produce video frames and audio samples at a particular rate.
+If video synchronization causes
+the emulated console to run, say, 5% faster than intended,
+that means audio samples are also being produced 5% faster.
+You might not notice the changed game speed,
+but you'll almost certainly notice
+the game's audio glitching constantly
+as your sound card tries to keep up.
+
+Enabling
+[audio synchronization](interface/higan.md#the-settings-menu)
+normally fixes this kind of audio glitching,
+but with video synchronization it makes things worse:
+audio is likely to glitch
+while higan waits for a video frame to be shown,
+and video is likely to stutter
+while higan waits for an audio buffer to complete.
+
+This game is running too fast!
+------------------------------
+
 TODO
 
-Why do Synchronize Video and Synchronize Audio conflict?
---------------------------------------------------------
-
-where is the vsync option, and the video/audio rate tradeoff
-https://board.byuu.org/viewtopic.php?p=43305#p43305
-
 games run in fast-forward if there's no audio device
 https://board.byuu.org/viewtopic.php?p=44138#p44138
 
-exclusive full-screen?
-
-phones and tablets
+Can I run higan on a phone or tablet?
+-------------------------------------