heatmap.js: JSDoc
heatmap.js: JSDoc

Convert comments to JSDoc and add code examples.
Remove "NOTE" on `getValueAt` which is redundant due to types.

benliddicott/DefinitelyTyped,georgemarshall/DefinitelyTyped,zuzusik/DefinitelyTyped,magny/DefinitelyTyped,alexdresko/DefinitelyTyped,AgentME/DefinitelyTyped,rolandzwaga/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,zuzusik/DefinitelyTyped,chrootsu/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,arusakov/DefinitelyTyped,laurentiustamate94/DefinitelyTyped,arusakov/DefinitelyTyped,AgentME/DefinitelyTyped,alexdresko/DefinitelyTyped,borisyankov/DefinitelyTyped,dsebastien/DefinitelyTyped,zuzusik/DefinitelyTyped,arusakov/DefinitelyTyped,AgentME/DefinitelyTyped,markogresak/DefinitelyTyped,one-pieces/DefinitelyTyped,AgentME/DefinitelyTyped,rolandzwaga/DefinitelyTyped,magny/DefinitelyTyped,chrootsu/DefinitelyTyped,dsebastien/DefinitelyTyped