docs: Clarify what the certificate is supposed to be in the saml config.
docs: Clarify what the certificate is supposed to be in the saml config.

Closes #17955.

Users were sometimes confused about what this certificate is supposed to
be.

zulip/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,punchagan/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip