Clarify MatrixBase<Derived>::applyOnTheLeft() documentation

The current documentation renders like this:

replaces *this by other * *this

(Closest I can get it in GitLab) ... which looks a lot like B^*A (equiv. B^HA), which wouldn't make the most sense for applyOnTheLeft(), but it took me longer than it should have to finally convince myself it is saying this:

replaces *this by other × *this

This could be achieved (in Doxygen) by using × when an asterisk is used for multiplication, and not adjoint or dereference. (It's doing a lot of work here.)

Alternatively something like this:

Calling A.applyOnTheLeft(B) replaces A with BA.

uses more natural notation without overloaded *s.