Button: Repeat limitations within option descriptions

[scottgonzalez]

The docs currently say:

When using an input of type button, submit or reset, support is limited to plain text labels with no icons.

We should repeat this within the descriptions for the affected options.

[jzaefferer]

This applies to current docs, so I'll do a PR against master, then later rebase the 1-12 branch. Shouldn't conflict with changes on button docs.