[crypto] Post-merge review: add sensitive data buffer doc (#24585)

Add documentation to SensitiveData[Fixed]Buffer clases.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

src/crypto/CHIPCryptoPAL.h

@@ -228,6 +228,12 @@ class ECPKey
                                                      const Sig & signature) const                                              = 0;
 };
 
+/**
+ * @brief Helper class for holding sensitive data that should be erased from memory after use.
+ *
+ * The sensitive data buffer is a variable-length, fixed-capacity buffer class that securely erases
+ * the contents of a buffer when the buffer is destroyed.
+ */
 template <size_t kCapacity>
 class SensitiveDataBuffer
 {
@@ -291,6 +297,12 @@ class SensitiveDataBuffer
     size_t mLength = 0;
 };
 
+/**
+ * @brief Helper class for holding fixed-sized sensitive data that should be erased from memory after use.
+ *
+ * The sensitive data buffer is a fixed-length, fixed-capacity buffer class that securely erases
+ * the contents of a buffer when the buffer is destroyed.
+ */
 template <size_t kCapacity>
 class SensitiveDataFixedBuffer
 {