Should whitespace be syntactically important in `u_str` macro?

I didn't find any example of products of units with u_str in the docs. There are lots of examples like `4u"m/s"` but it would be nice to see something like `3u"N*m"`. I kept trying `3u"N m"` expecting it to work.

Start by reading the existing u_str documentation and examples, especially the 4u"m/s" form, then inspect how products such as N*m are represented. Clarify whether whitespace in 3u"N m" is supported or should be documented as invalid, and update the relevant example or guidance when the expected behavior is established.

Written by the indexing model from the issue text.