Added a page with limitations, starting with quote escapes

docs/index.rst

@@ -29,6 +29,7 @@ PLC Doc's documentation!
    src/directives
    src/examples
    src/modules
+   src/limitations
 
 
 ##################

docs/src/limitations.rst

@@ -0,0 +1,49 @@
+######################
+Limitations and Issues
+######################
+
+For an up-to-date insight on issues, see https://github.com/DEMCON/plcdoc .
+Please help maintaining this project by adding any new issues there.
+
+Grammar
+=======
+
+Grammar refers to the parsing of PLC code.
+This is done through a language specification, custom made in TextX.
+And so there is no guarantee that all possible PLC code can be parsed without error.
+Nonetheless, much of the most occurring code will parse with no problems.
+
+Expressions
+-----------
+
+With expressions we mean all the literally typed constructions, from a simple ``5`` or ``'Hello World!'`` to a more complex ``CONCAT(INT_TO_STRING(5 + 3 * 8), '...')``.
+
+Currently, expressions are not parsed recursively as they would be for a real interpreter.
+Instead, when an expression is expected the whole item is simply matched as a string.
+So in the following...
+
+.. code-block::
+
+   my_int      : INT := 1 + 1;
+   my_string   : STRING := CONCAT("+", MY_CONST);
+
+...the initial values are registered as just ``"1 + 1"`` and ``"CONCAT("+", MY_CONST)"``.
+This means e.g. variables are never recognized in variable initialization and won't be linked.
+Aside from this, the expression should be printed normally in your generated docs item.
+
+String Escape
+-------------
+
+Because `expressions <#Expressions>`_ are typically matched until the next ``;``, this breaks when a literal semicolon appears in a string.
+This is avoided specifically for singular string expressions, but not for any expression:
+
+.. code-block::
+
+   str1  : STRING := 'Hi;';               // This will parse
+   str2  : STRING := CONCAT('Hi', ';');   // This will cause a parsing error
+
+Workaround
+^^^^^^^^^^
+
+For now you might have to use ``$3B`` as a `string constant <https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_plc_intro/2529327243.html&id=>`_ as a replacement for the literal semicolon.
+Or introduce (const) variables to break up those expressions.