SI-7961 Fix false positive procedure warnings

Two issues are fixed in this commit:
 - `def foo: Unit` was detected as missing a return type
 - The warning was emitted for constructors, but
   `def this(...): Unit = ...` is not valid Scala syntax
Added information on how to launch and debug scalac inside Eclipse
Merge pull request #3145 from dotta/issue/update-eclipse-readme

Added information on how to launch and debug scalac inside Eclipse
fix typo
Merge pull request #3141 from soc/SI-7961

SI-7961 Fix false positive procedure warnings
Merge pull request #3151 from xuwei-k/future-typo

fix typo