docs(cheatsheet): add q:/q/?, cmdline <C-f>, and :helpgrep

CHEATSHEET.md

@@ -18,6 +18,15 @@ Default Vim/Neovim bindings and commands worth remembering.
 - `gq{motion}`: format word-wrap over `{motion}`
 - `{Visual}gq`: format word-wrap of the visual selection
 - `gqq`: format word-wrap of the current line
+- `q:`: open the command-line window (edit and re-run past `:` commands;
+  `<CR>` executes the line, `:q` closes)
+- `q/` / `q?`: same, for forward / backward search history
+
+## Cmdline mode
+
+- `<C-f>`: open the command-line window for the cmdline being typed
+  (`'cedit'`, default `<C-f>`). Same window as `q:` but reached without
+  leaving cmdline mode.
 
 ## Commands
 
@@ -25,6 +34,8 @@ Default Vim/Neovim bindings and commands worth remembering.
 
 - `:make`: run `makeprg`, put output in the quickfix list
 - `:grep`: run `grepprg`, put results in the quickfix list
+- `:helpgrep {pattern}`: search all help files, put matches in the quickfix
+  list
 - `:cex {expr}`: build a quickfix list from `{expr}` and jump to the first
   entry, e.g. `:cex system('make')`
 - `:cgete {expr}`: like `:cex` but don't jump to the first entry