Fix misc typos, mostly in comments.

A collection of typos I happened to spot while reading code, as well as
grepping for common mistakes.

Backpatch to all supported versions, as applicable, to avoid conflicts
when backporting other commits in the future.

src/backend/executor/nodeModifyTable.c

@@ -1347,7 +1347,7 @@ lreplace:;
  * (but still lock row, even though it may not satisfy estate's
  * snapshot).
  *
- * Returns true if if we're done (with or without an update), or false if
+ * Returns true if we're done (with or without an update), or false if
  * the caller must retry the INSERT from scratch.
  */
 static bool