update IsValidSignatureEncoding doc
update IsValidSignatureEncoding doc

Summary:
update IsValidSignatureEncoding documentation, removing hashtype
from the signature description

Test Plan: n/a

Reviewers: #bitcoin_abc, deadalnix, matiu

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2095

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