Documentation/x86: Introduce enclave runtime management section

Enclave runtime management is introduced following the pattern
of the section describing enclave building. Provide a brief
summary of enclave runtime management, pointing to the functions
implementing the ioctl()s that will contain details within their
kernel-doc.

Signed-off-by: Reinette Chatre <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Link: https://lkml.kernel.org/r/1da0b9a[e-mail]

Documentation/x86/sgx.rst

···
                sgx_ioc_enclave_init
                sgx_ioc_enclave_provision
 
+Enclave runtime management
+--------------------------
+
+Systems supporting SGX2 additionally support changes to initialized
+enclaves: modifying enclave page permissions and type, and dynamically
+adding and removing of enclave pages. When an enclave accesses an address
+within its address range that does not have a backing page then a new
+regular page will be dynamically added to the enclave. The enclave is
+still required to run EACCEPT on the new page before it can be used.
+
+.. kernel-doc:: arch/x86/kernel/cpu/sgx/ioctl.c
+   :functions: sgx_ioc_enclave_restrict_permissions
+               sgx_ioc_enclave_modify_types
+               sgx_ioc_enclave_remove_pages
+
 Enclave vDSO
 ------------