Merge #12434: [doc] dev-notes: Members should be initialized

Summary:
fa9461473 [doc] dev-notes: Members should be initialized (MarcoFalke)

Pull request description:

Also, remove mention of threads that were removed long ago.

Motivation:
Make it easier to spot bugs such as #11654 and  #12426

Tree-SHA512: 8ca1cb54e830e9368803bd98a8b08c39bf2d46f079094ed7e070b32ae15a6e611ce98d7a614f897803309f4728575e6bc9357fab1157c53d2536417eb8271653

Partial (mostly) backport of Core PR12434
https://github.com/bitcoin/bitcoin/pull/12434/files

This diff brings this section of the developer-notes up-to-date with recent backporting efforts (~mid 2018) but retains the -Wshadow documentation applicable to ABC.
It includes some documentation additions from https://github.com/bitcoin/bitcoin/pull/8794/files#diff-e4de6d71249c78e229a61d6a03ac255a
and skips backporting https://github.com/MarcoFalke/bitcoin/commit/f203ecc621c0af594c43c7fce8a765a8259e3b43

Test Plan:
make
Read it.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3207