chore: fix limit and limitWeighted Scaladoc

[pjfanning]

Motivation

The Scaladoc of limit and limitWeighted does not match the code:

• the javadsl Flow, Source, SubFlow and SubSource versions were copied
  from take: they say the stream completes when "the defined number of
  elements has been taken" and completes without producing elements
  when n is zero or negative, but these operators fail the stream with
  StreamLimitReachedException once the limit is exceeded
• the scaladsl limitWeighted "Completes when" line talks about the
  number of emitted elements rather than the accumulated cost
• all of them name the exception StreamLimitException, which does not
  exist; LimitWeighted fails with StreamLimitReachedException

Modification

Describe the actual behaviour in the Emits/Completes/Errors/Cancels
lines of limit and limitWeighted in scaladsl Flow and javadsl Flow,
Source, SubFlow and SubSource, drop the copied zero-or-negative
sentence, add the missing Errors lines and name
StreamLimitReachedException.

Result

The Scaladoc matches the LimitWeighted stage (limit is limitWeighted
with a cost of 1 per element).

Tests

• Not run - Scaladoc only

References

Refs #3590

[He-Pin]

The StreamLimitReachedException fix and the removal of the zero/negative sentence are both correct against Ops.scala and StreamLimitReachedException.scala, and DSL parity looks complete. One thing still open before I merge: the published operator docs still carry the exact defect you are fixing here.