collection: fixed Collection[E]::first commentary

Signed-off-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
collection: Improved the description of Container[E]

Signed-off-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
collection : Fix the description of Set[E] class

Signed-off-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Merge branch 'collection doc' of https://github.com/BlackMinou/nit