Documentation/powerpc/fadump: add additional parameter feature details

Update the fadump document to include details about the fadump
additional parameter feature.

The document includes the following:
- Significance of the feature
- How to use it
- Feature restrictions

No functional changes are introduced.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250123114254.200527-5-sourabhjain@linux.ibm.com