[clang-format] Fix documentation for AllowAllParametersOfDeclarationOnNextLine

The current description of AllowAllParametersOfDeclarationOnNextLine in
the Clang-Format Style Options guide suggests that it is possible to
format function declaration, which fits in a single line (what is not
supported in current clang-format version). Also the example was not
reproducible and mades no sense.

Patch by Lucja Mazur, thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312721 91177308-0d34-0410-b5e6-96231b3b80d8