add documentation for symlink

Taking it off the secret menu :)
Merge pull request #7542 from sjkelly/patch-1

add documentation for symlink
Define det and logdet for AbstractMatrix. Fix some unrelated ambiguity warnings triggered by that change.
Explain symlink() behavior under WinXP. Ref #7542
Merge pull request #7544 from JuliaLang/anj/det

Define det and logdet for AbstractMatrix.