Clarify how `FOG_CREDENTIAL` env var is used
Clarify how `FOG_CREDENTIAL` env var is used

This commit clarifies how the FOG_CREDENTIAL env var is used, providing examples of a .fog file and an example command.

gds-operations/vcloud-core,gds-operations/vcloud-core,UKHomeOffice/vcloud-core,UKHomeOffice/vcloud-core