Tweak translation docs for regenerating translation strings in out-of-tree builds

Summary: make translate works out-of-tree, yet still updates the source file in place. This isn't clear from the docs.

Test Plan:
cd build/src
make translate
bitcoin_en.ts is regenerated

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2661