Also, move PDU Session Release diagram to new PDU Session Management section in rrc-dev.md.

Changes:

PDU Session Modify:
- Update PDU session modification sequence diagram
- Add DRB-To-Remove/To-Modify/To-Setup list handling
- Document E1AP Bearer Context Modification flow
- Add RRC reconfiguration trigger after E1AP response

QoS Flows Handling:

- Complete overview with 3GPP standards references (TS 23.501, 37.324, 38.463, 29.281, 38.331)
- Comprehensive Mermaid sequence diagram covering control and data plane

Technical details:
- Multiple QoS flows per DRB supported
- QFI to DRB mapping at RRC and SDAP layer
- F1-U tunnels: 1 per DRB, no QFI marking
- N3 tunnels: 1 per PDU session, with QFI marking
Signed-off-by: Guido Casati <guido.casati@openairinterface.org>