Trainer documentation fixes

[sfilipi]

MatrixFactorizationTrainer

• [x] collaborative filtering needs to link to an explanation.
• [x] key-typed should be "key type".
• [ ] low-rank factor matrix needs a link.

OlsTrainer

• [x] ordinary least squares needs a link.

OnlineGradientDescentTrainer

• [x] Online Gradient Descent (OGD) needs a link.
• [x] "// This trainer is not numerically stable. Please see issue #2425. "Should no appear twice in the example.

LinearSvm

• [x] Has no samples.
• [x] Has no proper description of what it does.

LbfgsMaximumEntropyMulticlassTrainer

• [ ] errors in equations
• [x] "no reource" typo

SdcaMaximumEntropyMulticlassTrainer

• [x] Scoring function has ill rendered equations.
• [x] The SdcaMulticlassTrainerBase in the "Training Algorithm Details" has a broken link, that tries to search for an app in the store upon clicking.

SdcaMulticlassTrainerBase

• [x] equations are not properly rendered.

[sfilipi]

Closed with #3575 and #3565