docs: document hilbish.vimAction hook

docs/hooks/hilbish.txt

@@ -1,3 +1,7 @@
 + `hilbish.exit` > Sent when Hilbish is about to exit.
 
-+ `hilbish.vimMode` > Sent when Hilbish's Vim mode is changed (example insert to normal mode)
++ `hilbish.vimMode` -> modeName > Sent when Hilbish's Vim mode is changed (example insert to normal mode),
+`modeName` is the name of the mode changed to (can be `insert`, `normal`, `delete` or `replace`).
+
++ `hilbish.vimAction` -> actionName, args > Sent when the user does a "vim action," being something
+like yanking or pasting text. See `doc vimMode actions` for more info.