Add basic instructions for writing custom filter functions to docs

[Saithe6]

Atm it's very difficult to figure out how to write a custom function for filtering the taglist (I think the tasklist might have filtering too? Idk I don't use it) when it's ultimately pretty simple. For those who're wondering, the function has to take an argument that represents the tag and return true if it should be shown or false if it should be hidden.

[Aire-One]

The doc from the git version is a more detailed and includes a description of the callback constraints: https://awesomewm.org/apidoc/widgets/awful.widget.taglist.html#filter. Ultimately, this is a pretty common pattern for a filter function

[gabrielfrimodig]

As @Aire-One mentioned, the documentation from git version is quite detailed.

You can add a custom function to filters the tags like this:

local function tag_filter(tag)
   return #tag:clients() > 0
end

This function takes a single argument (tag object) and return true if the tag should be shown and false otherwise. If the tag has clients.

To apply the filter, set the filter property:

my_taglist = awful.widget.taglist {
    screen  = s,
    filter  = tag_filter,
    buttons = taglist_buttons
}