tcg/tcg.h: Improve documentation of TCGv_i32 etc types

The typedefs we use for the TCGv_i32, TCGv_i64 and TCGv_ptr
types are somewhat confusing, because we define them as
pointers to structs, but the structs themselves are never
defined. Explain in the comments a bit more clearly why
this is OK and what is going on under the hood.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1477067922-26202-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>