Base: Update boot_parameters(7) to describe the enable_ioapic option

The document describes the implications of enabling and disabling that
option on the ability to enable SMP mode, and describes the requirements
for enabling IOAPIC mode even without enabling SMP mode.

Base/usr/share/man/man7/boot_parameters.md

@@ -35,6 +35,12 @@ List of options:
 
 * **`disable_virtio`** - If present on the command line, virtio devices will not be detected, and initialized on boot.
 
+* **`enable_ioapic`** - This parameter expects **`on`** or **`off`** and is by default set to **`on`**.
+  When set to **`off`**, the kernel will initialize the two i8259 PICs.
+  When set to **`on`**, the kernel will try to initialize the IOAPIC (or IOAPICs if there's more than one),
+  but only if **`acpi`** is set to **`limited`** or **`on`**, and a `MADT` (APIC) table is available.
+  Otherwise, the kernel will fallback to use the i8259 PICs.
+
 * **`fbdev`** - This parameter expects **`on`** or **`off`**.
 
 * **`force_pio`** - If present on the command line, the IDE controllers will be force into PIO mode when initialized IDE Channels on boot.
@@ -53,11 +59,12 @@ List of options:
 * **`pci_ecam`** - This parameter expects **`on`** or **`off`**.
 
 * **`root`** - This parameter configures the device to use as the root file system. It defaults to **`/dev/hda`** if unspecified.
-  
+
 * **`smp`** - This parameter expects a binary value of **`on`** or **`off`**. If enabled kernel will
-  use [APIC](https://en.wikipedia.org/wiki/Advanced_Programmable_Interrupt_Controller) mode
-  for handling interrupts instead of [PIC](https://en.wikipedia.org/wiki/Programmable_interrupt_controller) mode.
-  This parameter defaults to **`off`**.
+  enable available APs (application processors) and use them with the BSP (Bootstrap processor) to
+  schedule and run threads.
+  This parameter defaults to **`off`**. This parameter requires **`enable_ioapic`** to be enabled
+  and and a `MADT` (APIC) table to be available.
 
 * **`system_mode`** - This parameter is not interpreted by the Kernel, and is made available at `/proc/system_mode`. SystemServer uses it to select the set of services that should be started. Common values are:
   - **`graphical`** (default) - Boots the system in the normal graphical mode.