More documentation

Adds documentation for:

-   parent scope, observer semantics, binder semantics, syntax
-   with scope, observer semantics, binder semantics
-   logical and and or observer, binder, binder semantics
-   default observer, syntax
-   defined observer, binder
-   concat semantics
-   last semantics
-   only observer semantics, binder semantics
-   every and some block binder semantics
-   polymorphic extensibility

Removes the grammar section. `grammar.pegjs` should suffice now.

Fixes references to the old content change listener interface, now range
change listeners.

Fixes range and map content syntax and binder semantics.

Adds to and qualifies the arguments of known operators in the syntax
description.
Merge pull request #21 from kriskowal/doc-scope

More documentation