Update expressions doc to clarify what's observed

[arthurevans]

• In most cases (verify this) when an identifier is used in an expression, we
  observe the identifier and its direct properties.

For example, in {{ catalog }} we observe catalog and catalog.item but
not catalog.item.title.

• For object literals, we only observe the top-level identifiers.

For example, in: {{ { thing1: foo, thing2: bar } }} we observe foo and bar but not the properties of foo and bar.

• Also we need to add array access to the list of allowable syntax in expression.

For example: thing[field]. In this case, we observe... (fill in the blank).

[arthurevans]

Sorry, opened this on the wrong repo. Moved to:

Polymer/old-docs-site#392