Merge pull request #114 from tzz/master

More docs for hard/soft classes and xrefs.
Forward porting of 133699ee5e69e from the cfengine core repository.

This documentation was made while documentation was still in core.

Originally written by Michael V. Pelletier <mvpel@yahoo.com>.
Use prototype and attributes macros
Merge pull request #115 from kacfengine/sys_uptime

Forward porting of 133699ee5e69e from the cfengine core repository.
Add history tags for new variables
Fix incorrect instructions for integrating git into update policy
Merge pull request #117 from nickanderson/fix/git-integration-instructions

Fix incorrect instructions for integrating git into update policy
Add passphraseless key generation information
Some people seem to have issues with key generation. This makes it more
precise.
Add note about other authentication possibilities
Improve instructions for mission portal ssh key install
Fix permissions typo
Merge pull request #119 from nickanderson/fix/git-integration-instructions

Add passphraseless key generation information
Remove unhelpful tags from overview pages
Clarify warn_only behavior: all such promises are not kept