Add getRevision to api

The revision can be useful for instance to check if a feature has been modified so I'd like to see it in api.
Merge pull request #2245 from sweco-sebhar/patch-1

Make `ol.Observable#getRevision` exportable.
Add symbol kind to info
Merge pull request #2246 from tschaub/kind

Add symbol kind to info.
Fix broken links in api docs landing page
Merge pull request #2252 from probins/patch-1

Fix broken links in api docs landing page
Use string values for ol.source.State enum

Many user-facing enums are string values and we document use of the string values rather than more confusing enum reference or the numeric values.
Merge pull request #2247 from pagameba/source-state

Use string values for ol.source.State enum
Add ol.easing to api docs
Merge pull request #2256 from fredj/easing.jsdoc

Add the jsdoc ol.easing namespace