Docs: Clarify that Profile Small doesn't cover all the platforms

Add notes in build instruction document to clairfy that Profile
Small config doesn't all the platforms supported in TF-M.

Change-Id: I5cc94e02175276d1954505e68e89321c0a418a48
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/user_guides/tfm_build_instruction.rst b/docs/user_guides/tfm_build_instruction.rst
index 3f5d0f9..9637633 100644
--- a/docs/user_guides/tfm_build_instruction.rst
+++ b/docs/user_guides/tfm_build_instruction.rst
@@ -331,10 +331,10 @@
 | PsaApiTestIPCTfmLevel2 | IPC        | 2             | No              | No             | No            | Yes               | TF-M & PSA API tests    |
 +------------------------+------------+---------------+-----------------+----------------+---------------+-------------------+-------------------------+
 | DefaultProfileS        | Library    | 1             | No              | No             | No            | No                | TF-M Profile Small,     |
-|                        |            |               |                 |                |               |                   | no tests                |
+|                        |            |               |                 |                |               |                   | no tests [7]_           |
 +------------------------+------------+---------------+-----------------+----------------+---------------+-------------------+-------------------------+
 | RegressionProfileS     | Library    | 1             | Yes             | Yes            | No            | No                | TF-M Profile Small &    |
-|                        |            |               |                 |                |               |                   | Regression tests        |
+|                        |            |               |                 |                |               |                   | Regression tests [7]_   |
 +------------------------+------------+---------------+-----------------+----------------+---------------+-------------------+-------------------------+
 
 .. [1] Which TF-M Model is set: `CORE_IPC`. If `CORE_IPC` is set to true then the model is IPC model, otherwise it's library model.
@@ -349,6 +349,9 @@
 
 .. [6] Build for PSA API compliance tests or not: `PSA_API_TEST`.
 
+.. [7] Profile Small config doesn't cover all the platforms. Please check
+       Profile Small config files to find out the supported platforms.
+
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