thumb: Clarify a comment about the conversions that are done

The comment made it look like the condition code was dropped from
the extra add instruction, while it actually was handled properly.

Thus, the comment was misleading while the code itself did the right
thing.

Also clarify the comment indicating that we use the full three-operand
form of the add instruction.

Change-Id: I2c1ac6ac4fedf262d104ea30a6c005febc74de9c

build/make/thumb.pm

@@ -51,7 +51,7 @@ sub FixThumbInstructions($$)
 
     # Convert register post indexing to a separate add instruction.
     # This converts "ldrneb r9, [r0], r2" into "ldrneb r9, [r0]",
-    # "add r0, r2".
+    # "addne r0, r0, r2".
     s/^(\s*)((ldr|str)(ne)?[bhd]?)(\s+)(\w+),(\s*\w+,)?\s*\[(\w+)\],\s*(\w+)/$1$2$5$6,$7 [$8]\n$1add$4$5$8, $8, $9/g;
 
     # Convert a conditional addition to the pc register into a series of