Existence and usage of Numeric is non-obvious due to the lack of any documentation

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The usage and existance of Numeric might be obvious for experts, but there seems to be not a single bit of information about it on the web page:

• Numeric's ScalaDoc is more or less non-existing, apart from some trivial method descriptions.
• Not a single word on docs.scala-lang.org.

Considering that this is one of the few, very common patterns existing in the wild and the questions about it are quite frequent, this should be covered in detail.

Existing documentation could maybe taken from the improved Numeric's implementation here: https://github.com/azavea/numeric

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Imported From: https://issues.scala-lang.org/browse/SI-5222?orig=1 Reporter: @soc

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@adriaanm said: Unassigning and rescheduling to M6 as previous deadline was missed.