Include link to UAST Parse Examples & XPath Reference

[ricardobaeta]

I would be pretty happy if a UAST Parse Examples URL could be included on the cli output when running srcd parse --help. It would look something like:

Classify languages, parse files, and manage parsers

Usage:
  srcd parse [command]

Examples: 
https://docs.sourced.tech/engine/#babelfish-uast (examples)

Available Commands:
  drivers     Manage language drivers.
  lang        Identify the language of the given files.
  uast        Parse and return the filtered UAST of the given file(s)

Flags:
  -h, --help   help for parse

Global Flags:
      --config string   config file (default is $HOME/.srcd.yaml)
  -v, --verbose         if true, log all of the things

Use "srcd parse [command] --help" for more information about a command.

Moreover, I think it would be great to have some feedforward on XPath Syntax errors, like:

could not stream: rpc error: code = Unknown desc = could not apply query //uast:Identifier/Name: UastFilter() failed: Undefined namespace prefix
XPath Syntax Reference https://doc.bblf.sh/using-babelfish/uast-querying.html

I know we might be moving to css selectors, but let's cherish what we have now.