DOC: Document members in header methods

[jhlegarreta]

Document members in header methods.

Transfer the documentation from the ivar or the method definition to the
header methods where the documentation was incomplete or missing.

Take advantage of the commit to:

• Improve the syntax of the existing documentation to stick to the
  recommendations of the ITK SW Guide to generate the Doxygen
  documentation (i.e. use the /** <documentation.> */ convention).
• Improve the style of the existing documentation (e.g. capitals,
  punctuation) to stick to the recommendations of the ITK SW Guide.
• Improve the wording of some comments.
• Remove the documentation of methods that are either not documented as
  per the ITK SW Guide (e.g. constructors) or which are inherited members
  whose purpose is not modified (e.g. PrintSelf).
• Remove the comments used to mark the end of a method, which are not
  considered as necessary by the ITK SW Guide.
• Remove empty documentation blocks.
• Move the private member methods before the variables.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)

[jhlegarreta]

Note that this change does not mean to be exhaustive at all. I just came across these cases while I was working on #2327. Thanks.

[dzenanz]

Please rebase to resolve conflicts.

[jhlegarreta]

Please rebase to resolve conflicts.

Done.