doc: Updated outdated help command for getblocktemplate
Summary of Changes
- Removed coinbasetxn from the help outputs
- Added the missing name for transactions in the help outputs
- Added help outputs for longpollid and default_witness_commitment
- Added more clarity to capabilities, rules, and coinbaseaux
Rationale
The outputs from the help command for getblocktemplate are outdated and don't reflect the actual results from getblocktemplate (see #19625 (closed) for more details)
Fixes #19625 (closed).