Update docs.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

Author: sjaeckel

doc/crypt.tex

@@ -9124,6 +9124,12 @@ \subsection{LTC\_SMALL\_CODE}
 When this is defined some of the code such as the Rijndael and SAFER+ ciphers are replaced with smaller code variants.
 These variants are slower but can save quite a bit of code space.
 
+\subsection{LTC\_SMALL\_STACK}
+When this is defined some of the code uses a variant which results in smaller stack sizes.
+Depending on the architecture and other configuration options the results of execution speeed can vary.
+Therefore we try to enable this automatically where it brings an advantage in speed.
+In case you always want smaller stack usage, no matter if it makes the execution slower, you should enable this.
+
 \subsection{LTC\_PTHREAD}
 When this is activated all of the descriptor table functions will use pthread locking to ensure thread safe updates to the tables.  Note that
 it doesn't prevent a thread that is passively using a table from being messed up by another thread that updates the table.

src/headers/tomcrypt_custom.h

@@ -146,6 +146,9 @@
 /* Use small code where possible */
 /* #define LTC_SMALL_CODE */
 
+/* Always use small stack sizes where possible */
+/* #define LTC_SMALL_STACK */
+
 /* clean the stack of functions which put private information on stack */
 /* #define LTC_CLEAN_STACK */