Bug 37198: Improve GetPreparedLetter documentation

Bug 37198: Improve GetPreparedLetter documentation

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 149412cb62a074ccdef1e1c2bbbd2bee35c48498)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 84814bbef33ac9c04b12cb3f063b2a11cfd0b2ce)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: wainuiwitikapark <wainuiwitikapark@catalyst.net.nz>
(cherry picked from commit 021191bfb65bfc174f0385891d0e6ebab0106ca3)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>