CTR_DRBG: more consistent formatting and wording

In particular, don't use #MBEDTLS_xxx on macros that are undefined in
some configurations, since this would be typeset with a literal '#'.
diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h
index 5955079..17a5cfc 100644
--- a/include/mbedtls/ctr_drbg.h
+++ b/include/mbedtls/ctr_drbg.h
@@ -21,11 +21,11 @@
  *   nonce (see mbedtls_ctr_drbg_seed()).
  *
  * Note that the value of #MBEDTLS_CTR_DRBG_ENTROPY_LEN defaults to:
- * - \c 48 if the module #MBEDTLS_SHA512_C is enabled and the symbol
- *   #MBEDTLS_ENTROPY_FORCE_SHA256 is not enabled at compile time.
+ * - \c 48 if the module \c MBEDTLS_SHA512_C is enabled and the symbol
+ *   \c MBEDTLS_ENTROPY_FORCE_SHA256 is not enabled at compile time.
  *   This is the default configuration of the library.
- * - \c 32 if the module #MBEDTLS_SHA512_C is disabled at compile time.
- * - \c 32 if #MBEDTLS_ENTROPY_FORCE_SHA256 is enabled at compile time.
+ * - \c 32 if the module \c MBEDTLS_SHA512_C is disabled at compile time.
+ * - \c 32 if \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled at compile time.
  */
 /*
  *  Copyright (C) 2006-2019, Arm Limited (or its affiliates), All Rights Reserved
@@ -85,7 +85,7 @@
 /** The amount of entropy used per seed by default.
  *
  * This is 48 bytes because the entropy module uses SHA-512
- * (#MBEDTLS_ENTROPY_FORCE_SHA256 is not set).
+ * (\c MBEDTLS_ENTROPY_FORCE_SHA256 is not set).
  *
  * \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
  *       acceptable.
@@ -95,7 +95,8 @@
 /** The amount of entropy used per seed by default.
  *
  * This is 32 bytes because the entropy module uses SHA-256
- * (the SHA512 module is disabled or #MBEDTLS_ENTROPY_FORCE_SHA256 is set).
+ * (the SHA512 module is disabled or
+ * \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).
  *
  * \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
  *       acceptable.