[backport#14741] doc: Indicate -rpcauth option password hashing alg
Summary:
dcb70b152292a8df9fe834bacdf231ea848819b1 Indicate -rpcauth option password hashing alg (Carl Dong)
Pull request description:
By indicating the password hashing algorithm, users of bitcoin distributions without the script in `share/rpcauth` and users who don't want to rely on said script can use alternative means to generate the password hash. Question for reviewers: perhaps we should also indicate that it is specifically a HMAC-SHA-256 of the _**UTF-8**_ encoding of their password?
Backport of Core PR14741
Test Plan:
ninja
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D8131