Merge #11698: [Docs] [Qt] RPC-Console nested commands documentation

Merge #11698: [Docs] [Qt] RPC-Console nested commands documentation

c3055bb Add help-console command to Qt debug console (Luke Mlsna)

Pull request description:

  This PR would close issue #9195 by adding documentation for the debug console features (mainly nested commands) which were added in [PR #7783](https://github.com/bitcoin/bitcoin/pull/7783).

  The following changes were made to QT debug console code:
  - Added a line to the initial message text at the top of the debug console:

  > For more information on using this console type **help-console**.

  - Added a pseudo-command `help-console` which is hooked after parsing the request, but before actually executing the RPC thread. It prints the following text to the console as if it were a valid RPC response.

  > This console accepts RPC commands using the standard syntax.
  >    example:    getblockhash 8
  > This console can also accept RPC commands using bracketed syntax.
  >    example:    getblockhash(8)
  > A space or a comma can be used to separate arguments for either syntax.
  >    example:    sendtoaddress \<address\> \<amount\>
  >                    sendtoaddress,\<address\>,\<amount\>
  > Commands may be nested when specified with the bracketed syntax.
  >    example:    getblockinfo(getblockhash(0),true).
  > Result values can be queried with a non-quoted string in brackets.
  >    example:    getblock(getblockhash(0) true)[height]

  This seemed like a reasonably sane way to introduce a fake RPC help command, but

Tree-SHA512: 35d73dcef9c4936b8be99e80978169f117c22b94f4400c91097bf7e0e1489060202dcd738d9debdf4c8a7bd10709e2c19d4f625f19e47c4a034f1d6019c0e0f2