Add more details to the certification path building documentation

Add more details to the certification path building documentation

Added more details about the certification path building algorithm,
especially about the behavior in case of incomplete chains in the trust
store.

Fixes #29681

Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
MergeDate: Tue Mar 24 17:24:15 2026
(Merged from https://github.com/openssl/openssl/pull/30317)

(cherry picked from commit 6a5fea7ec5df1c8cc4bb5006013301b41d0accd4)