tril(M, k) not documented

[jiahao]

The two-argument forms of tril, tril!, triu, and triu! exist for some matrix types but are not documented. Furthermore, coverage over the special matrix types is spotty.

[jakebolewski]

tril and triu also do not do any bounds checking for out of range diagonals.

[kshyatt]

In particular, these special matrix types don't work with triu and tril (basically all the ones in Base.LinAlg):

• Bidiagonal
• Tridiagonal
• SymTridiagonal
• Hermitian
• UpperTriangular
• LowerTriangular
• Diagonal

[ViralBShah]

A lot of stuff doesn't work on special matrix types. I tend to use them sparingly, and only in linear algebra calls, and not much otherwise.

[jiahao]

See #136

[kshyatt]

Finished by JuliaLang/julia#12574.

[tkelman]

Still missing docs. nevermind, sorry, you must've done that a while back