Android NumberPicker binding documentation

[tbalcom]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Docs update

⤵️ What is the current behavior?

The recently added Android.Widget.NumberPicker bindings are not yet documented.

🆕 What is the new behavior (if this is a feature change)?

Adds documentation for the recently added Android.Widget.NumberPicker Value and DisplayedValues bindings.

I added a Notes column to the table because the order of the bindings are important; if Value is declared before DisplayedValues then the NumberPicker will always start at 0. This happens because the default value of NumberPicker.Max starts at 0. When the Value binding is declared first then NumberPicker.SetValue() is being called before NumberPicker.Max has been set. I can't think of any reasonable way around this.

I also fixed a minor bug in the MvvmCross.Droid.Support.V7.Preference table which was missing a column header for the Mvx version introduced column.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

N/A

📝 Links to relevant issues/docs

N/A

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated (docs style guide)
• Rebased onto current develop