doc: add some more info on cmp-vimtex

refer: #2818

doc/vimtex.txt

@@ -4592,6 +4592,21 @@ knows how to install and configure plugins, though!
 [2]: https://github.com/hrsh7th/cmp-omni
 
 nvim-cmp with cmp-vimtex ~
+
+`cmp-vimtex` is a dedicated `nvim-cmp` extension that gives some extra
+benefits compared to the more general `cmp-omni`:
+
+* Allows fuzzy matching against all info provided by Vimtex (including
+ bibliographic details, useful for citations).
+* Parses and display all details contained in bibtex files for cite
+ completions.
+* Trims long strings in the completion menu (to add space for the documenation
+ window).
+* Triggers the completion menu automatically after typing `\cite{`.
+* Allows more granular configuration of the menus, and more.
+
+To use `cmp-vimtex`, you should add something like the following to your
+configuration. For more info, check the README of the github repo ([1] above).
 >lua
  local cmp = require('cmp')
  cmp.setup {