docs(guide/interpolation): add known issue (do not change the content of interpolated strings)

[gkalpak]

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

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
Closes #12813

[Narretz]

So are known issues things we don't want to fix? To me it sounds like we are aware and maybe gonna fix it.
In the guide I would give the section a more descriptive name or group the section about embedded interpolation together with this one under Known Issues

[Narretz]

To clarify, known issue sounds like something that can maybe be fixed, but this here is difficult to fix because it's how the framework works. And we don't recommend a workaround we simply say "don't do this"

[gkalpak]

For me, known issues are things that are indeed not working as expected, but are either too difficult to fix or would require a considerable breaking change or for whatever reason there is no plan to fix them any time soon.

I am fine documenting this particular one as a limitation (something that is not supported and should not be used).

[Narretz]

I think we are not making a definitive decision here wheter this is considered a bug or simply expected behavior. So let's leave it as is - only thing is, can you

give the section a more descriptive name and group the section about embedded interpolation together with this one under "Known Issues"

?