Update 6.9-to-7.0.md

fix gitlab.yml example diff command
Text changes to projects API documentation

Fixes some grammar / punctuation errors and inconsistencies.
Fixed typos in Development - Rake Tasks doc
Merge pull request #7411 from robbytaylor/patch-3

Fixed typos in Development - Rake Tasks doc
Merge pull request #7410 from robbytaylor/patch-1

Text changes to projects API documentation
Merge pull request #7201 from nacengineer/patch-2

Update 6.9-to-7.0.md
Add manage group role to group master