docs(datepicker): does not follow Material Design guidelines for messages

[Splaktar]

Actual Behavior:

• What is the issue? *
  The tone in the date picker validations example does not fit within the Material Design guidelines.
• What is the expected behavior?
  It should follow the Writing Tone Guidelines as people are likely to reproduce this example in their apps.

CodePen (or steps to reproduce the issue): *

• CodePen Demo which shows your issue: https://codepen.io/Splaktar/pen/zPVwEM
• Details: Messages like "Date is too early!" don't follow the Material Design guidelines

AngularJS Versions: *

• AngularJS Version: 1.6.6
• AngularJS Material Version: 1.1.5

Additional Information:

• Browser Type: * Chrome
• Browser Version: * 62
• OS: * macOS 10.13.2
• Stack Traces: N/A

I just ran into a client today who pointed out that they were just using similar messages to those found in the Material Design examples.

[HZooly]

Hi @Splaktar!

What would you imagine for min & max date that could follow MD guidelines?

[Splaktar]

@TorzuoliH "Choose a date on or after <min>" and "Choose a date on or before <max>".