[css-typed-om] add detailed descriptions of the rest of the methods on StylePropertyMap

[nainar]

[nainar]

@tabatkins

[shans]

This comprises the following list:

• get a value from a StylePropertyMap
• set a value on a StylePropertyMap
• getAll
• has
• delete

[nainar]

The last one in this list was pull request 451. Closing this issue now.