add documentation explaining compound filetypes

lua/lspconfig/util.lua

@@ -21,7 +21,14 @@ M.default_config = {
 -- global on_setup hook
 M.on_setup = nil
 
----@param filetype string the filetype to check (can be a compound, dot-separated filetype; see |'filetype'|)
+---Test if a given filetype string matches any of the expected filetypes
+---
+---Note that vim filetypes can be a dot-separated string containing multiple filetypes, and the plugins/syntax rules for
+---all sub-filetypes are applied to the buffer. e.g. a buffer with a filetype of `yaml.ansible` will load the ftplugin
+---and syntax rules for both `yaml` and `ansible` (and in that order). see `:h 'filetype'`
+---
+---Accordingly, this will attempt to match any of the sub-filetypes against the expected filetype(s)
+---@param filetype string the filetype to check (can be a compound, dot-separated filetype)
 ---@param expected string|string[] the filetype(s) to match against
 ---@return boolean
 function M.ft_matches(filetype, expected)