docs(treesitter): improve 'no parser' error message for InspectTree

Improve error messages for `:InspectTree`, when no parsers are available
for the current buffer and filetype. We can show more informative and
helpful error message for users (e.g., which lang was searched for):

```
 ... No parser available for the given buffer:
+... no parser for 'custom_ft' language, see :help treesitter-parsers
```

Also improve the relevant docs for *treesitter-parsers*.

runtime/lua/vim/treesitter/language.lua

@@ -119,6 +119,10 @@ local function ensure_list(x)
 end
 
 --- Register a parser named {lang} to be used for {filetype}(s).
+---
+--- Note: this adds or overrides the mapping for {filetype}, any existing mappings from other
+--- filetypes to {lang} will be preserved.
+---
 --- @param lang string Name of parser
 --- @param filetype string|string[] Filetype(s) to associate with lang
 function M.register(lang, filetype)