docs: add system use case with retry as alternate flow

docs/use-cases/system-use-cases.md

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+### Use case: Enroll contactless card to receive transit benefit
+
+**Primary Actor**: Transit rider
+**Systems**: Benefits app, payment processor
+
+**Preconditions**:
+
+- Transit rider has confirmed their eligibility with the Benefits app
+- Transit rider has their contactless card information available
+- Benefits app is able to contact the payment processor
+
+**Trigger**: Transit rider initiates the enrollment process
+
+**Basic flow**:
+
+1. Transit rider enters their contactless card information
+2. Benefits app passes that information to the payment processor to enroll the card
+3. Payment processor successfully enrolls card
+
+**Alternate flows**:
+
+- 3a. Payment processor returns with one of the following errors: address verification failed, token is invalid, or general server error
+    - 3a1. Transit rider chooses to retry, starting back at initiating the enrollment process
+    - 3b1. Transit rider leaves the Benefits app
+
+**Postconditions**:
+
+- Transit rider's contactless card is enrolled to receive the transit benefit