Document basic A_mul_B! methods

Also removes deprecated A_mul_B documentation.

Closes #5862.

Note: this does _not_ cover all the currently defined methods for
A_mul_B!, many of which do _not_ have method signatures of this form.
Ref: #5332

created via:
  git diff d9b1f39~ d9b1f39 | sed 's/base/math/g' | git apply
  git add doc/stdlib/math.rst
  git commit -C d9b1f39

[av skip]

Author: jiahao

doc/stdlib/math.rst

@@ -318,9 +318,19 @@ Mathematical Operators
 
    Matrix operator A \\ B\ :sup:`T`
 
-.. function:: A_mul_B(...)
+.. function:: A_mul_B!(Y, A, B) -> Y
 
-   Matrix operator A B
+   Calculates the matrix-matrix or matrix-vector product *A B* and stores the
+   result in *Y*, overwriting the existing value of *Y*.
+
+   .. doctest::
+
+      julia> A=[1.0 2.0; 3.0 4.0]; B=[1.0 1.0; 1.0 1.0]; A_mul_B!(B, A, B);
+
+      julia> B
+      2x2 Array{Float64,2}:
+       3.0  3.0
+       7.0  7.0
 
 .. function:: A_mul_Bc(...)