Docs: Add disable-wallet section to OSX build instructions

Summary:
And update line in Unix instructions.

The disable-wallet section was mentioned in the Berkeley DB section of
the OSX build instructions, but the section did not actually exist. This
PR ports the section from the Unix build instructions.

Backport of core PR14153
https://github.com/bitcoin/bitcoin/pull/14153/files

Test Plan: Read the docs.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3925