(doc) MNGSITE-353: Document maven.repo.local property

[AElMehdi]

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[AElMehdi]

@hboutemy,
As discussed before I changed the documentation of the org.apache.maven.model.interpolation.StringSearchModelInterpolator based on what it existed in the previous maven version.
I tried to figure out from the code if it still existed/used which apparently is, since it's referenced in some tests/classes!
Thanks.

[michael-o]

Something is wrong here: <<<$\{${user.home}/.m2/repository\}>>>. Too many braces and dollars.

[AElMehdi]

I removed the redundant ones.