docs: Add documentation on new cmake/configure options

docs/INSTALL_AUTOTOOLS

@@ -285,7 +285,7 @@ Some ./configure options deserve additional comments:
 
 	libssh2 can use the Libgcrypt library
 	(https://www.gnupg.org/) for cryptographic operations.
-	Either Libgcrypt or OpenSSL is required.
+        One of the cryptographic libraries is required.
 
 	Configure will attempt to locate Libgcrypt
 	automatically.
@@ -299,7 +299,7 @@ Some ./configure options deserve additional comments:
 
 	libssh2 can use the OpenSSL library
 	(https://www.openssl.org) for cryptographic operations.
-	Either Libgcrypt or OpenSSL is required.
+        One of the cryptographic libraries is required.
 
 	Configure will attempt to locate OpenSSL in the
 	default location.
@@ -307,6 +307,20 @@ Some ./configure options deserve additional comments:
 	If your installation of OpenSSL is in another
 	location, specify it using --with-libssl-prefix.
 
+ * --with-mbedtls
+ * --without-mbedtls
+ * --with-libmbedtls-prefix=[DIR]
+
+        libssh2 can use the mbedTLS library
+        (https://tls.mbed.org) for cryptographic operations.
+        One of the cryptographic libraries is required.
+
+        Configure will attempt to locate mbedTLS in the
+        default location.
+
+        If your installation of mbedTLS is in another
+        location, specify it using --with-libmbedtls-prefix.
+
  * --with-libz
  * --without-libz
  * --with-libz-prefix=[DIR]

docs/INSTALL_CMAKE

@@ -12,6 +12,7 @@ following cryptography libraries:
 * OpenSSL
 * Libgcrypt
 * WinCNG
+* mbedTLS
 
 Getting started
 ---------------
@@ -48,8 +49,8 @@ The following options are available:
 
     Chooses a specific cryptography library to use for cryptographic
     operations.  Can be `OpenSSL` (https://www.openssl.org),
-    `Libgcrypt` (https://www.gnupg.org/), `WinCNG` (Windows Vista+) or
-    blank to use any library available.
+    `Libgcrypt` (https://www.gnupg.org/), `WinCNG` (Windows Vista+),
+    `mbedTLS` (https://tls.mbed.org/) or blank to use any library available.
 
     CMake will attempt to locate the libraries automatically.  See [2]
     for more information.
@@ -97,6 +98,10 @@ The following options are available:
 
     Will enable the libssh2_trace() function for showing debug traces.
 
+ * `CLEAR_MEMORY=ON`
+
+    Securely zero memory before freeing it (if the backend supports this).
+
 Build tools
 -----------