Update changelog and migration guide

This reflect changes to the config version symbols.

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
diff --git a/ChangeLog.d/split-config.txt b/ChangeLog.d/split-config.txt
index 7fe2955..9a1c678 100644
--- a/ChangeLog.d/split-config.txt
+++ b/ChangeLog.d/split-config.txt
@@ -5,8 +5,12 @@
      change the build configuration, and should generally only be included from
      build_info.h.
    * The handling of MBEDTLS_CONFIG_FILE has been moved into build_info.h.
-   * Mandatory version symbols MBEDTLS_CONFIG_VERSION and
-     MBEDTLS_USER_CONFIG_VERSION were introduced for MBEDTLS_CONFIG_FILE and
-     MBEDTLS_USER_CONFIG_FILE respectively. They have to be defined with a
-     value of one in their respective config file for the config to be
-     considered valid.
+   * Config file symbols MBEDTLS_CONFIG_VERSION and MBEDTLS_USER_CONFIG_VERSION
+     were introduced for use in MBEDTLS_CONFIG_FILE and
+     MBEDTLS_USER_CONFIG_FILE respectively.
+     Defining them to a particular value will ensure that mbedtls interprets
+     the config file in a way that's compatible with the config file format
+     indicated by the value.
+     The config file versions are based on the value of MBEDTLS_VERSION_NUMBER
+     of the mbedtls version that first introduced that config file format.
+     The only value currently supported is 0x03000000.
diff --git a/docs/3.0-migration-guide.d/split_config.md b/docs/3.0-migration-guide.d/split_config.md
index 8fba5b3..fc80925 100644
--- a/docs/3.0-migration-guide.d/split_config.md
+++ b/docs/3.0-migration-guide.d/split_config.md
@@ -9,8 +9,12 @@
 into `build_info.h`, including the handling of the `MBEDTLS_CONFIG_FILE`
 macro.
 
-Mandatory version symbols were introduced for `MBEDTLS_CONFIG_FILE` and
-`MBEDTLS_USER_CONFIG_FILE`, `MBEDTLS_CONFIG_VERSION` and
-`MBEDTLS_USER_CONFIG_VERSION` respectively. Both config files should include
-a definiton of their respective version symbol, with a value of `1` to be
-considered valid.
+Config file symbols `MBEDTLS_CONFIG_VERSION` and `MBEDTLS_USER_CONFIG_VERSION`
+were introduced for use in `MBEDTLS_CONFIG_FILE` and
+`MBEDTLS_USER_CONFIG_FILE` respectively.
+Defining them to a particular value will ensure that mbedtls interprets
+the config file in a way that's compatible with the config file format
+indicated by the value.
+The config file versions are based on the value of `MBEDTLS_VERSION_NUMBER`
+of the mbedtls version that first introduced that config file format.
+The only value currently supported is `0x03000000`.