Document SingularException throw for inv(::AbstractMatrix) (#1331)

I suggest documenting that `inv(::AbstractMatrix)` throws on
(numerically) singular matrices, this is different from `inv(::Number)`
that returns `Inf`.

```julia 
inv( 0. )    # Inf, no exception
inv( [0;;] ) # SingularException 
```

I guess documenting this would mean that `inv` guarantees to return a
matrix with only finite (`isfinite`) entries if the argument has only
finite entries, but I don't know if the implementation guarantees it /
if this should be tested ?

Author: ViralBShah

src/generic.jl

@@ -1152,6 +1152,8 @@ Matrix inverse. Computes matrix `N` such that
 Computed by solving the left-division
 `N = M \\ I`.
 
+A [`SingularException`](@ref) is thrown if `M` fails numerical inversion.
+
 # Examples
 ```jldoctest
 julia> M = [2 5; 1 3]