Update function.dd

Sentence now starts with capital letter. Also corrected reason for disallowed overloading for non-D linkage. 

It is true that C name mangling does NOT take parameter types into account with the consequence that there is no function overloading in C. However, C++ name mangling DOES mentions parameter types and enables function overloading in this way. Therefore, the statement "name mangling does not take the parameter types into account" is not correct for all linkage types.
Merge pull request #441 from ralphtandetzky/patch-11

Update function.dd