[Processing] GRASS7 provider: review and update all r.* description files.

[ghost]

Description

I have manually reviewed all GRASS 7.2 raster algorithms and I have fixed lots of description files.
All GRASS raster algorithms should now use the latest parameters from GRASS 7.2.

I still need to work on vector and imagery description files (for next week) before completing this work but now that GRASS7 Processing provider is back into master, I believe I should publish this PR as soon as possible.

All new/modified parameters are syntactically correct because Processing checks them whenever the provider is launched and report errors in the MessageLog.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and containt sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[nyalldawson]

Lovely work!

[timlinux]

Nice! BTW I am still unable to run any GRASS algs on MacOS... I will share the error message later on the ML....