Removes `experimental` from `experimental_fixed_format_kernels` flag

Renames `experimental_fixed_format_kernels` build option to
`fixed_format_kernels`.

Adds documentation for the flag covering basics:
- What fixed-format kernels are
- Why they're needed
- Which backend they're for (i.e. CPU)
- Some pointers on how to use them.

Resolves: ONCPUML-1253
Change-Id: I428c98614c309c9ffc32d0f32daa24740f7cb967
Signed-off-by: Nathan John Sircombe <nathan.sircombe@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/9523
Benchmark: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: SiCong Li <sicong.li@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/tests/SConscript b/tests/SConscript
index 8596cfa..33f709e 100644
--- a/tests/SConscript
+++ b/tests/SConscript
@@ -1,7 +1,7 @@
 #!/usr/bin/python
 # -*- coding: utf-8 -*-
 
-# Copyright (c) 2017-2022 Arm Limited.
+# Copyright (c) 2017-2023 Arm Limited.
 #
 # SPDX-License-Identifier: MIT
 #
@@ -185,7 +185,7 @@
         test_env['LINKFLAGS'].append('-static-openmp')
 
 # Testing for fixed format GEMM kernels.
-if env['experimental_fixed_format_kernels'] and test_env['validation_tests']:
+if env['fixed_format_kernels'] and test_env['validation_tests']:
     test_env.Append(CPPDEFINES = ['ARM_COMPUTE_ENABLE_FIXED_FORMAT_KERNELS'])
 
 if test_env['validation_tests']: