Update backupstoragelocation.md SSE-C

[MrOffline77]

• During setup of SSE-C I discovered strange behavior and unclear documentation on how to configure SSE-C
• I read the code where SSE-C came in Add SSE-C encryption support #96 and updated the description so other people get a chance to understand how this works. The Author of Add SSE-C encryption support #96 implemented this for Helm Charts as far as I understood, but when using velero install instead of helm, things get confusing on how to setup SSE-C properly.
• I assumed that the referenced file customerKeyEncryptionFile transforms into a secret when used with velero install like the secret-file file which holds the S3 creds. But instead its referencing the path to the secret from the within the velero pod.