Tighten the comments on the mpeg PRX changes

Cut restatement and asides that only made sense against earlier, wrong versions
of the code, and prefer parentheses over paired dashes. Also fix two comments
left stale by the descriptor rework, and record the rule in AGENTS.md.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -252,6 +252,17 @@ private:
 
 But generally follow the surrounding style. Braces are preferred on the same line. Braces are always used even when they could be omitted due the inner part being just a single line.
 
+### Comments
+
+Keep comments tight. Say the thing once; don't restate what the code already shows, and don't
+allude to a previous, now-corrected version ("eight addresses and nothing else" - the "and nothing
+else" only makes sense against the old wrong layout, which is history). Cut filler like "the two
+are the same shape" down to "(the same shape)".
+
+For a parenthetical aside, prefer parentheses over a pair of spaced dashes: write "the audio thread
+that paces playback", or "the audio thread (which paces playback)", not "the audio thread - which
+paces playback -". A single trailing dash to tack on an example is fine.
+
 We've been inconsistent with copyright notices, but for new files, have the year at 2012, and add the "This program is free software..." as in other files.
 
 `// Copyright (c) 2012- PPSSPP Project.`