Update the PdfDocument docs with unsupported operations.

The PDF generating canvas we use does not yet support all
drawing operations. This change adds docs which operations
are not yet supported.

Change-Id: I8d5ee8c1e2a473ba783b5e62f23ab90a54bbf106
(cherry picked from commit 8db45b844b5482cf61839404c0f6847e053de817)
1 file changed