Clarify docs

Author: kuhar

mlir/docs/PatternRewriter.md

@@ -327,9 +327,12 @@ that locally have the most benefit. The benefit of a pattern is decided solely
 by the benefit specified on the pattern, and the relative order of the pattern
 within the pattern list (when two patterns have the same local benefit).
 
+The driver performs a post-order traversal. Note that it walks regions of the
+given op but does not visit the op.
+
 This driver does not (re)visit modified or newly replaced ops, and does not
 allow for progressive rewrites of the same op. Op erasure is only supported for
-the currently matched op. If your pattern-set requires these, consider using the
+the currently matched op. If your pattern set requires these, consider using the
 Greedy Pattern Rewrite Driver instead, at the expense of extra overhead.
 
 This driver is exposed using the `walkAndApplyPatterns` function.