Add comment explaining the ctxt field in Span

Author: nzig

src/libsyntax_pos/span_encoding.rs

@@ -60,8 +60,10 @@ const CTXT_INDEX: usize = 2;
 
 // Tag = 0, inline format.
 // -----------------------------------
-// | base 31:8  | len 7:1  | tag 0:0 |
+// | base 31:8  | len 7:1  | ctxt (currently 0 bits) | tag 0:0 |
 // -----------------------------------
+// Since there are zero bits for ctxt, only SpanData with a 0 SyntaxContext
+// can be inline.
 const INLINE_SIZES: [u32; 3] = [24, 7, 0];
 const INLINE_OFFSETS: [u32; 3] = [8, 1, 1];