configfs-tsm-report: Document size limits for outblob attributes

The configfs-tsm-report interface can fail with -EFBIG when the
attestation report generated by a TSM provider exceeds internal
maximums (TSM_REPORT_OUTBLOB_MAX). However, this error condition and
its handling are not currently documented in the ABI.

Userspace tools need to understand how to interpret various error
conditions when reading attestation reports.

Document that reads fail with -EFBIG when reports exceed size limits,
with guidance on how to resolve them.

Reviewed-by: Fang Peter <peter.fang@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Link: https://patch.msgid.link/20260211001712.1531955-2-sathyanarayanan.kuppuswamy@linux.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: Kuppuswamy Sathyanarayanan

Documentation/ABI/testing/configfs-tsm-report

@@ -17,6 +17,12 @@ Description:
 		where the implementation is conveyed via the @provider
 		attribute.
 
+		This interface fails reads and sets errno to EFBIG when the
+		report generated by @provider exceeds the configfs-tsm-report
+		internal maximums. Contact the platform provider for the
+		compatible security module, driver, and attestation library
+		combination.
+
 What:		/sys/kernel/config/tsm/report/$name/auxblob
 Date:		October, 2023
 KernelVersion:	v6.7
@@ -31,6 +37,9 @@ Description:
 		Standardization v2.03 Section 4.1.8.1 MSG_REPORT_REQ.
 		https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/specifications/56421.pdf
 
+		See "EFBIG" comment in the @outblob description for potential
+		error conditions.
+
 What:		/sys/kernel/config/tsm/report/$name/manifestblob
 Date:		January, 2024
 KernelVersion:	v6.10
@@ -43,6 +52,9 @@ Description:
 		See 'service_provider' for information on the format of the
 		manifest blob.
 
+		See "EFBIG" comment in the @outblob description for potential
+		error conditions.
+
 What:		/sys/kernel/config/tsm/report/$name/provider
 Date:		September, 2023
 KernelVersion:	v6.7