Add a “Use” section to the <content> docs

[chasenlehara]

It should include some examples, like this component:

Component.extend( {
    tag: "my-tag",
    view: stache( "<h1><content>Hello world</content></h1>" )
} );

Used like this:

<my-tag></my-tag>

Renders:

<my-tag><h1>Hello world</h1></my-tag>

While this:

<my-tag>Hello friend</my-tag>

Renders:

<my-tag><h1>Hello friend</h1></my-tag>

[chasenlehara]

It’d also be nice to refer people to slots/templates when they want to be able to have multiple pieces of content inserted into their template.