doxygen: fix parameter name typos and misused \p commands

\p is reserved for function parameters.
\c is used to describe other values and variables.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index ec83551..0e9d58a 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -395,7 +395,7 @@
  *
  * \return          0 on success (signature is valid),
  *                  #MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if there is a valid
- *                  signature in sig but its length is less than \p siglen,
+ *                  signature in \p sig but its length is less than \p sig_len,
  *                  or a specific error code.
  *
  * \note            For RSA keys, the default padding type is PKCS#1 v1.5.
@@ -459,7 +459,7 @@
  *                  #MBEDTLS_ERR_PK_TYPE_MISMATCH if the PK context can't be
  *                  used for this type of signatures,
  *                  #MBEDTLS_ERR_PK_SIG_LEN_MISMATCH if there is a valid
- *                  signature in sig but its length is less than \p siglen,
+ *                  signature in \p sig but its length is less than \p sig_len,
  *                  or a specific error code.
  *
  * \note            If hash_len is 0, then the length associated with md_alg