Add help-console command to Qt debug console

Summary:

• Added help-console to the list of autocompletion strings
• Implemented requested changes to help message:
  • Added an example that uses access-by-index getblock(getblockhash(0) true)[tx][0]
  • Replace "bracketed syntax" to "parenthesized syntax" where applicable
  • Replace "separate" with "delimit"
  • Removed <br> and <b>help/help-console</b> from translation strings, since these parts don't change between languages
  • Changed examples to be based off getblock 0 so they will work even with pruned/no blockchain and disablewallet if copied and pasted
  • Clarified syntax for queries of named/unnamed result objects.

Backport of Core PR #11698
Completes T430
Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: ninja check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1938