add GenericDocument<>::Swap with std::swap() support

This avoids the dependency on the <algorithm> header, as suggested by
@miloyip in #376.

Make it usable with old VC8 (VS2005)

Add #pragma intrinsic(_umul128) for MSVS 2005

Add #pragma intrinsic(_umul128) for MSVS 2005

Add GenericDocument ctor overload to specify JSON type.

See #368.

Update FAQ 8 with shorter version of clean-and-minimize idiom

Credits to @pah who suggested it in comment to #368.

@pah recommended to mark this constructor as explicit to avoid accidentally creating a temporary GenericDocument from a Type enum value (because all arguments but the first one are optional).

Add to FAQ: How to clear-and-minimize a document?

Closes #368

Update also documentation of the existing GenericDocument constructor.

It unifies the interfaces with Value where kXXXType can be passed
into constructor.
It enables shortcut that helps to avoid extra SetXXX() call following
construction of a document.

Add missing allocator to uses of AddMember

AddMember requires allocator, this fix keeps all uses of AddMember consistent across the whole tutorial.

Fix warnings when compiling with clang and -Wimplicit-fallthrough