[myanmar] Document Medials

https://github.com/harfbuzz/harfbuzz/pull/3648#discussion_r894532676

src/hb-ot-shaper-indic.hh

@@ -88,10 +88,10 @@ enum ot_category_t {
   OT_D0      = 34,	// Digit zero
   OT_DB      = OT_N,	// Dot below
   OT_GB	     = OT_PLACEHOLDER,
-  OT_MH      = 35,	// Medial
-  OT_MR      = 36,	// Medial
-  OT_MW      = 37,	// Medial
-  OT_MY      = 38,	// Medial
+  OT_MH      = 35,	// Medial Ha, Mon La
+  OT_MR      = 36,	// Medial Ra
+  OT_MW      = 37,	// Medial Wa, Shan Wa
+  OT_MY      = 38,	// Medial Ya, Mon Na, Mon Ma
   OT_PT      = 39,	// Pwo and other tones
   OT_VS      = 40,	// Variation selectors
   OT_P       = 41,	// Punctuation

src/hb-ot-shaper-myanmar-machine.rl

@@ -81,10 +81,10 @@ export VPst = 23;
 export As   = 32;	# Asat
 export D    = 33;	# Digits except zero
 export D0   = 34;	# Digit zero
-export MH   = 35;	# Medial
-export MR   = 36;	# Medial
-export MW   = 37;	# Medial
-export MY   = 38;	# Medial
+export MH   = 35;	# Medial Ha, Mon La
+export MR   = 36;	# Medial Ra
+export MW   = 37;	# Medial Wa, Shan Wa
+export MY   = 38;	# Medial Ya, Mon Na, Mon Ma
 export PT   = 39;	# Pwo and other tones
 export VS   = 40;	# Variation selectors
 export P    = 41;	# Punctuation