Clarify Comments and Fix Typos (#651)

Fixes many typos, and errors in comments.

* Clarifies many comments
* Grammar correction in config.pl help text
* Removed comment about MBEDTLS_X509_EXT_NS_CERT_TYPE.
* Comment typo fix (Dont => Don't)
* Comment typo fix (assure => ensure)
* Comment typo fix (byes => bytes)
* Added citation for quoted standard
* Comment typo fix (one complement => 1's complement)

The is some debate about whether to prefer "one's complement",  "ones'
complement", or "1's complement".  The more recent RFCs related to TLS
(RFC 6347,  RFC 4347, etc) use " 1's complement", so I followed that
convention.

* Added missing ")" in comment
* Comment alignment
* Incorrect comment after #endif
diff --git a/library/pkparse.c b/library/pkparse.c
index 275429e..efdf437 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -1187,12 +1187,12 @@
 #endif /* MBEDTLS_PEM_PARSE_C */
 
     /*
-    * At this point we only know it's not a PEM formatted key. Could be any
-    * of the known DER encoded private key formats
-    *
-    * We try the different DER format parsers to see if one passes without
-    * error
-    */
+     * At this point we only know it's not a PEM formatted key. Could be any
+     * of the known DER encoded private key formats
+     *
+     * We try the different DER format parsers to see if one passes without
+     * error
+     */
 #if defined(MBEDTLS_PKCS12_C) || defined(MBEDTLS_PKCS5_C)
     if( ( ret = pk_parse_key_pkcs8_encrypted_der( pk, key, keylen,
                                                   pwd, pwdlen ) ) == 0 )