Update and correct documentation of extended data types (xtypes).

Documentation/usage/icarus_verilog_extensions.rst

@@ -7,20 +7,24 @@ standard. Some of these are picked from extended variants of the
 language, such as SystemVerilog, and some are expressions of internal
 behavior of Icarus Verilog, made available as a tool debugging aid.
 
-* Builtin System Functions
+Built-in System Functions
+-------------------------
 
-** Extended Verilog Data Types
+Extended Verilog Data Types
+---------------------------
 
-This feature is turned off if the generation flag "-g" is set to other
-then the default "2x".  For example, "iverilog -g2x" enables extended
-data types, and "iverilog -g2" disables them.
+This feature is turned on by the generation flag "-gxtypes" and turned
+off by the generation flag "-gno-xtypes". It is turned on by default.
 
 Icarus Verilog adds support for extended data types. This extended
 type syntax is based on a proposal by Cadence Design Systems,
-originally as an update to the IEEE1364. That original proposal has
-apparently been absorbed by the IEEE1800 SystemVerilog
-standard. Icarus Verilog currently only takes the new primitive types
-from the proposal.
+originally as an update to the IEEE 1364 standard. Icarus Verilog
+currently only takes the new primitive types from the proposal.
+
+SystemVerilog provides the same functionality using somewhat different
+syntax. This extension is maintained for backwards compatibility.
+
+- Types
 
 Extended data types separates the concept of net/variable from the
 data type. Both nets and variables can declared with any data
@@ -28,7 +32,7 @@ type. The primitive types available are::
 
     logic  - The familiar 0, 1, x and z, optionally with strength.
     bool   - Limited to only 0 and 1
-    real   - 64bit real values
+    real   - 64-bit real values
 
 Nets with logic type may have multiple drivers with strength, and the
 value is resolved the usual way. Only logic values may be driven to
@@ -61,9 +65,9 @@ types as nets.
 - Ports
 
 Module and task ports in standard Verilog are restricted to logic
-types. This extension removes that restriction, allowing any type to
-pass through the port consistent with the continuous assignment
-connectivity that is implied by the type.
+types. This extension removes that restriction, allowing any of
+the above types to pass through the port consistent with the
+continuous assignment connectivity that is implied by the type.
 
 - Expressions
 
@@ -81,5 +85,6 @@ bools and reals always return exactly true or false.
 Case comparison returns bool. This differs from baseline Verilog,
 which strictly speaking returns a logic, but only 0 or 1 values.
 
-All the arithmetic operators return bool if both of their operands are
-bool or real. Otherwise, they return logic.
+Arithmetic operators return real if either of their operands is real,
+otherwise they return logic if either of their operands is logic. If
+both operands are bool, they return bool.