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*.
fbe40caa7c
