Fix documentation of what functions restart when

The previous comment in ecp.h that only functions that take a "restart
context" argument can restart was wrong due to ECDH and SSL functions.
Changing that criterion to "document says if can return IN PROGRESS".

This requires updating the documentation of the SSL functions to mention this
explicitly, but it's something we really ought to do anyway, a bit
embarrassing that this wasn't done already - callers need to know what
`MBEDTLS_ERR_SSL_xxx` error codes to special-case. Note that the documentation
of the relevant functions was in a suboptimal state, so it was improved in the
process - it could use some more improvement, but only the changes that helped
cleanly insert the info about the IN_PROGRESS part were done here.

Also, while updating the ecp.h comment, I noticed several functions in the
ECDH module were wrongfully documented as restartable, which is probably a
left-over from the days before `mbedtls_ecdh_enable_restart()` was introduced.
Fixing that as well, to make the criterion used in ecp.h correct.

include/mbedtls/ecp.h

@@ -350,9 +350,19 @@ mbedtls_ecp_keypair;
  *                  same; they must not be used until the function finally
  *                  returns 0.
  *
- *                  This only affects functions that accept a pointer to a
- *                  \c mbedtls_ecp_restart_ctx as an argument, and only works
- *                  if that pointer is valid (in particular, not NULL).
+ *                  This only applies to functions whose documentation
+ *                  mentions they may return #MBEDTLS_ERR_ECP_IN_PROGRESS (or
+ *                  #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS for functions in the
+ *                  SSL module). For functions that accept a "restart context"
+ *                  argument, passing NULL disables restart and makes the
+ *                  function equivalent to the function with the same name
+ *                  with \c _restartable removed. For functions in the ECDH
+ *                  module, restart is disabled unless the function accepts
+ *                  an "ECDH context" argument and
+ *                  mbedtls_ecdh_enable_restart() was previously called on
+ *                  that context. For function in the SSL module, restart is
+ *                  only enabled for specific sides and key exchanges
+ *                  (currently only for clients and ECDHE-ECDSA).
  *
  * \param max_ops   Maximum number of basic operations done in a row.
  *                  Default: 0 (unlimited).