Spell out secure secret storage and sharing (#1875)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Travis Ralston <travpc@gmail.com>

changelogs/client_server/newsfragments/1875.clarification

@@ -0,0 +1 @@
+Document the acronyms and alternate names for the "Secrets" section.

content/client-server-api/modules/secrets.md

@@ -15,6 +15,9 @@ secret when storing, fetching, requesting, or sharing the secret.
 Secrets are plain strings; structured data can be stored by encoding it
 as a string.
 
+The mechanism described in this section is known as "secure secret storage and
+sharing", "SSSS", or "4S".
+
 #### Storage
 
 When secrets are stored on the server, they are stored in the user's