add descriptions of grep behavior

Author: haarg

ppcs/ppc0033-map-grep-with-topic.md

@@ -72,6 +72,12 @@ This will evaluate `BLOCK` for each element of `LIST`, aliasing `VAR` to each
 element. Its behavior will otherwise match `map BLOCK LIST`. It is only
 possible to use `my` variables for this.
 
+### `grep my VAR BLOCK LIST`
+
+This will evaluate `BLOCK` for each element of `LIST`, aliasing `VAR` to each
+element. Its behavior will otherwise match `grep BLOCK LIST`. It is only
+possible to use `my` variables for this.
+
 ### `map my (VAR, VAR) BLOCK LIST`
 
 This will evaluate `BLOCK` for each set of two elements in `LIST`, aliasing
@@ -82,6 +88,17 @@ On the last iteration, there may not be enough elements remaining to fill
 every `VAR` slot. In this case, a warning will be issued, and the extra `VAR`
 slots will be filled with `undef`.
 
+### `grep my (VAR, VAR) BLOCK LIST`
+
+This will evaluate `BLOCK` for each set of two elements in `LIST`, aliasing
+the `VAR` to the first of each set, and the second `VAR` to the second. More
+than two variables can be used to iterate over sets of three or more items.
+
+On the last iteration, there may not be enough elements remaining to fill
+every `VAR` slot. In this case, a warning will be issued, and the extra `VAR`
+slots will be filled with `undef`. The extra `undef` values used will also be
+included in the returned list.
+
 ## Backwards Compatibility
 
 As the syntax chosen is currently invalid, it should not present any backwards