Allow optionally giving an example to interpolation with i18n

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

It is not possible to specify an example for interpolation text.

Expected behavior

We should be able to specify an example sometimes. Renaming the variable to make more sense is fine too, or adding more context in

Minimal reproduction of the problem with instructions

<span i18n="saying hi">
  Hello, {{ name // i18n(ph="name", ex="John") }}
</span>

What is the motivation / use case for changing the behavior?

Our translators sometimes raise questions (these cause translations to be delayed) which could have been avoided if we had an example in the placeholder.

[JarekToro]

How does this work while translating an attribute? Is

Example:

 <span *ngIf='element.latestRecord'
 matTooltip='Currently Checked Out To {{element.latestRecord?.publisherName}}'
i18n-matTooltip="@@CurrentlyCheckedOutToTooltipPopover" matTooltipPosition='right'>

I have been unsuccessful in making a translation file that can handle the interpolation inside of an attribute.

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