Update our documentation on updating eSpeak-NG (#16978)
The docs on updating eSpeak-NG were slightly unclear, and had a couple of outdated things in them.
Description of user facing changes
None.
Description of development approach
Updated the eSpeak docs:
- Made it more obvious that as well as updating user_docs/en/changes.md, you also need to update projectDocs/dev/createDevEnvironment.md (this was already included, but it was not particularly obvious to me).
- Removed old references to the eSpeak version being in nvdaHelper/espeak/config.h, as it is now in nvdaHelper/espeak/sconstruct.
- Fixed up markdown formatting problems.
