Fix#3714:Improve Display Image Tutorial

For Bug #3714, user use the imshow() function without calling
namedWindow() properly, because user did not know the implicit rule for
imshow(). (i.e. without calling namedWindow implied calling namedWindow(
,CV__WINDOW_AUTOSIZE).

Improve the Display Image Tutorial to educate the user for such implied
rule.

Put a note on documentation for Bug#3714

Revise the documentation regarding to imshow() function to better
educate the user the implicit rule. (i.e. without creating the window
before the imshow() function is the same as creating AUTOSIZE window)
minor fix to make "updateBackgroundModel=false" option meaningful.
export thread control to generated wrappers
add extra to FeatureDetector docs
fix bug 2432 for 2.4 branch
Some changes to make migration to 3.0 easier
Merge pull request #3515 from StevenPuttemans:fix_FeatureDetector_docs
Merge pull request #3500 from StevenPuttemans:fix_2432_2.4
Merge pull request #3514 from mshabunin:compat-24
Merge pull request #3511 from mattip:2.4
Merge pull request #3484 from euphrat:gmg_update_fix
Merge pull request #3471 from HelenWong:FixBug3714
Minor grammar improvements to documentation