[CTI] Document options for controlling mechanism validation

Add documentation for the validate() function, and remove documentation
for the non-existent 'no_validation' flag.

doc/sphinx/cti/classes.rst

@@ -12,6 +12,8 @@ Basic Classes & Functions
 
 .. autofunction:: units
 
+.. autofunction:: validate
+
 .. autoclass:: state
    :no-undoc-members:
 

doc/sphinx/cti/input-files.rst

@@ -161,6 +161,18 @@ incorrect and would generate an error when processed::
     element(39.948, "Ar")                    # error
     element(symbol="Ar", 39.948)             # error
 
+Validation
+----------
+
+Normally, Cantera will make some checks for errors in the definitions of species
+and reactions, such as checking for duplicate reactions. To slightly speed up
+processing (if a mechanism has previously been validated), or in case of
+spurious validation errors, validation can be disabled using the
+:func:`validate` function. For example, to disable validation of reactions, add
+the following to the CTI file::
+
+    validate(reactions='no')
+
 .. _sec-dimensions:
 
 Dimensional Values

doc/sphinx/cti/phases.rst

@@ -259,9 +259,6 @@ or a sequence of strings from the table below.
 ==================================  ================
 Option String                       Meaning
 ==================================  ================
-``'no_validation'``                 Turn off all validation. Use when the definition
-                                    has been previously validated to speed up importing
-                                    the definition into an application. Use with caution!
 ``'skip_undeclared_elements'``      When importing species, skip any containing undeclared
                                     elements, rather than flagging them as an error.
 ``'skip_undeclared_species'``       When importing reactions, skip any containing undeclared

interfaces/cython/cantera/ctml_writer.py

@@ -254,6 +254,14 @@ def export_species(filename, fmt = 'CSV'):
     _valfmt = fmt
 
 def validate(species = 'yes', reactions = 'yes'):
+    """
+    Enable or disable validation of species and reactions.
+    :param species:
+        Set to 'yes' (default) or 'no'.
+    :param reactions:
+        Set to 'yes' (default) or 'no'. This controls duplicate reaction checks
+        and validation of rate expressions for some reaction types.
+    """
     global _valsp
     global _valrxn
     _valsp = species