[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!

llvm-svn: 312721