Incomplete sentence corrected. Resolves Issue #1997

[krutikabapat]

This pullrequest changes

• Resolves Issue FaceRecognizer::train docs seems to be incomplete #1997
• Corrects the documentation for FaceRecognizer::train() function.

FaceRecognizer::train(InputArrayOfArrays src, InputArray labels); 

The type InputArray can accept vector<T> or Mat types as mentioned here: https://docs.opencv.org/3.4/d4/d32/classcv_1_1__InputArray.html

Also added the way of using Mat for labels. (with type CV_32SC1)

force_builders=Docs

[alalek]

As a "bugfix" this patch should go into 3.4 branch first. We will merge changes from 3.4 into master regularly (weekly/bi-weekly).

So, please:

• change "base" branch of this PR: master => 3.4 (use "Edit" button near PR title)
• rebase your commits from master onto 3.4 branch. For example:
  git rebase -i --onto upstream/3.4 upstream/master
  (check list of your commits, save and quit (Esc + "wq" + Enter)
  where upstream is configured by following this GitHub guide and fetched (git fetch upstream).
• push rebased commits into source branch of your fork (with --force option)

Note: no needs to re-open PR, apply changes "inplace".

[krutikabapat]

@alalek - Thanks! I've changed the branch to 3.4 from master, re-based commits to 3.4 and force pushed the changes.