Trying to look up the help file for a Vim feature, but you cannot quite
remember the right keyword? Use :helpgrep. With :helpgrep, you can
search across all of the Vim help files for not just the specific keywords,
but any pattern of text. For instance, if you want to find where
substitution is mentioned in the help files, try:
:helpgrep substitution
It makes a list of all occurrences in the quick fix window and then opens up
a split with the cursor on the line of the first occurrence. You can then
hit :copen to see the rest of the entries.
See :h helpgrep for more details.