input placeholder and hint nomenclature is misleading

[ilovett]

https://material.io/guidelines/components/text-fields.html#text-fields-layout

MdPlaceholder acts like what should be the "Label" from the spec, perhaps rename it MdLabel

MdHint acts like what should be "Helper Text" from the spec, perhaps rename MdHelperText

"Placeholder Text (hint text)" from the spec appears to be unimplemented, AND conflicts with what I would assume should be namedMdPlaceholder and/or MdHint

[rafaelss95]

The MdPlaceholder part is a dupe of #6240.

About MdHint I really prefer the currnet name than MdHelperText.

[jelbourn]

Placeholder part is captured in #6240

We don't have any plans to change MdHint

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