documentation: fix tokens that can start indentation region

[brandonspark]

What:

The documentation currently states:

     *      The following tokens can start an indentation region:
     *
     *         :  =  =>  <-  if  then  else  while  do  try  catch  finally  for  yield  match

This is not actually true, however, if you look at the source of the predicate
which determines which tokens can start an indentation region: https://github.com/lampepfl/dotty/blob/865aa639c98e0a8771366b3ebc9580cc8b61bfeb/compiler/src/dotty/tools/dotc/parsing/Tokens.scala#L276

In particular, if I'm not mistaken, return, throw, and with are all also tokens which have this property.

This PR just amends the documentation to reflect that.

[brandonspark]

Can I have a new check? I've signed the CLA now.