docs(select): clarify behavior of multiple attribute

[Splaktar]

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Enhancement
[x] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

The docs are misleading and cause developers to think that something like multiple="ctrl.isMultiple" can be used. In fact, only the existence of the attribute matters. This follows a common pattern used in AngularJS Material APIs.

Issue Number:
Closes #3068

What is the new behavior?

• Align the multiple docs with the actual behavior of the API
• fix syntax ordering error with md-no-asterisk docs
• remove instances of multiple="multiple" from tests
• update test to check for non-existence of multiple rather than value
• add test to check for existence of multiple

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information