Difference between toLower() and asLowerCase() for strings?

[Jon D via Digitalmars-d-learn]

I'm trying to identify the preferred ways to lower case a string. 
In std.uni there are two functions that return the lower case 
form of a string: toLower() and asLowerCase(). There is also 
toLowerInPlace().

I'm having trouble figuring out what the relationship is between 
these, and when to prefer one over the other. Both take a 
strings, asLowerCase also takes range. Otherwise, I couldn't find 
the differences in the documentation. Implementations are 
apparently different, but not clear what the real difference is.

Are there reasons to prefer one over the other?

--Jon