Address PR review comments: expand pin signatures documentation

Updated based on PR #144 review feedback:

1. Expanded explanation of what pin signatures tell ChipFlow:
   - Physical pin connections
   - Simulation model and test bench selection
   - Pad and package pin allocation requirements

2. Added comprehensive IOModelOptions documentation:
   - Full reference of all available options
   - Examples showing basic and advanced usage
   - Details on invert, individual_oe, power_domain, clock_domain
   - Trip point options (CMOS, TTL, VCORE, VREF, SCHMITT_TRIGGER)
   - Buffer control and initialization options

This provides users with complete understanding of how to configure
pin electrical and behavioral properties.

Author: robtaylor

docs/using-pin-signatures.rst

@@ -27,9 +27,7 @@ Available Pin Signatures
 - ``QSPIFlashSignature()`` - Quad SPI flash interface
 - ``JTAGSignature()`` - JTAG debug interface
 
-All pin signatures support additional options:
-
-- ``sky130_drive_mode`` - Set drive strength for Sky130 (e.g., ``Sky130DriveMode.OPEN_DRAIN_STRONG_UP``)
+All pin signatures accept ``IOModelOptions`` to customize their electrical and behavioral properties (see below).
 
 Using Pin Signatures in Your Top-Level Design
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -49,7 +47,57 @@ Pin signatures are used when defining your top-level component's interface:
                 "flash": Out(QSPIFlashSignature()),
             })
 
-These signatures tell ChipFlow how to connect your design to the physical pins of your chip.
+These signatures tell ChipFlow:
+
+- How to connect your design to the physical pins of your chip
+- How to select appropriate simulation models and test benches for each interface
+- Requirements for pad and package pin allocation (power domains, drive strength, etc.)
+
+IO Model Options
+~~~~~~~~~~~~~~~~
+
+All pin signatures accept ``IOModelOptions`` to configure the electrical and behavioral properties of the I/O pins:
+
+.. code-block:: python
+
+    from chipflow_lib.platforms import GPIOSignature, IOTripPoint
+
+    super().__init__({
+        # Basic GPIO
+        "gpio_basic": Out(GPIOSignature(pin_count=4)),
+
+        # GPIO with custom options
+        "gpio_custom": Out(GPIOSignature(
+            pin_count=8,
+            invert=True,              # Invert all pins
+            individual_oe=True,       # Separate OE for each pin
+            clock_domain='io_clk',    # Use IO clock domain
+            trip_point=IOTripPoint.TTL,  # TTL input thresholds
+            init=0x00,                # Initial output values
+            init_oe=0xFF              # Initial OE values (all enabled)
+        ))
+    })
+
+Available IOModelOptions
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+- **invert** (``bool`` or ``Tuple[bool, ...]``) - Polarity inversion for pins. Can be a single bool for all pins or a tuple specifying inversion per pin.
+- **individual_oe** (``bool``) - If ``True``, each output wire has its own Output Enable bit. If ``False`` (default), a single OE bit controls the entire port.
+- **power_domain** (``str``) - Name of the I/O power domain. Pins with different power domains must be in separate signatures.
+- **clock_domain** (``str``) - Name of the I/O's clock domain (default: ``'sync'``). Pins with different clock domains must be in separate signatures.
+- **buffer_in** (``bool``) - Enable input buffer on the I/O pad.
+- **buffer_out** (``bool``) - Enable output buffer on the I/O pad.
+- **sky130_drive_mode** (:class:`Sky130DriveMode`) - Drive mode for Sky130 output buffers (see below).
+- **trip_point** (:class:`IOTripPoint`) - Input buffer trip point configuration:
+
+  - ``IOTripPoint.CMOS`` - CMOS switching levels (30%/70%) referenced to I/O power domain
+  - ``IOTripPoint.TTL`` - TTL levels (low < 0.8V, high > 2.0V)
+  - ``IOTripPoint.VCORE`` - CMOS levels referenced to core power domain
+  - ``IOTripPoint.VREF`` - CMOS levels referenced to external reference voltage
+  - ``IOTripPoint.SCHMITT_TRIGGER`` - Schmitt trigger for noise immunity
+
+- **init** (``int`` or ``bool``) - Initial values for output signals.
+- **init_oe** (``int`` or ``bool``) - Initial values for output enable signals.
 
 Sky130-Specific Pin Configuration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~