Move multi-clock to explanations (#1561)

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diff --git a/docs/src/explanations/multi-clock.md b/docs/src/explanations/multi-clock.md
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+---
+layout: docs
+title:  "Multiple Clock Domains"
+section: "chisel3"
+---
+Chisel 3 supports multiple clock domains as follows.
+
+Note that in order to cross clock domains safely, you will need appropriate synchronization logic (such as an asynchronous FIFO). You can use the [AsyncQueue library](https://github.com/ucb-bar/asyncqueue) to do this easily.
+
+```scala mdoc:silent:reset
+import chisel3._
+
+class MultiClockModule extends Module {
+  val io = IO(new Bundle {
+    val clockB = Input(Clock())
+    val resetB = Input(Bool())
+    val stuff = Input(Bool())
+  })
+
+  // This register is clocked against the module clock.
+  val regClock = RegNext(io.stuff)
+
+  withClockAndReset (io.clockB, io.resetB) {
+    // In this withClock scope, all synchronous elements are clocked against io.clockB.
+    // Reset for flops in this domain is using the explicitly provided reset io.resetB.
+
+    // This register is clocked against io.clockB.
+    val regClockB = RegNext(io.stuff)
+  }
+
+  // This register is also clocked against the module clock.
+  val regClock2 = RegNext(io.stuff)
+}
+```
+
+You can also instantiate modules in another clock domain:
+
+```scala mdoc:silent:reset
+import chisel3._
+
+class ChildModule extends Module {
+  val io = IO(new Bundle{
+    val in = Input(Bool())
+  })
+}
+class MultiClockModule extends Module {
+  val io = IO(new Bundle {
+    val clockB = Input(Clock())
+    val resetB = Input(Bool())
+    val stuff = Input(Bool())
+  })
+  val clockB_child = withClockAndReset(io.clockB, io.resetB) { Module(new ChildModule) }
+  clockB_child.io.in := io.stuff
+}
+```
+
+If you only want to connect your clock to a new clock domain and use the regular implicit reset signal, you can use `withClock(clock)` instead of `withClockAndReset`.
+
+```scala mdoc:silent:reset
+import chisel3._
+
+class MultiClockModule extends Module {
+  val io = IO(new Bundle {
+    val clockB = Input(Clock())
+    val stuff = Input(Bool())
+  })
+
+  // This register is clocked against the module clock.
+  val regClock = RegNext(io.stuff)
+
+  withClock (io.clockB) {
+    // In this withClock scope, all synchronous elements are clocked against io.clockB.
+
+    // This register is clocked against io.clockB, but uses implict reset from the parent context.
+    val regClockB = RegNext(io.stuff)
+  }
+
+  // This register is also clocked against the module clock.
+  val regClock2 = RegNext(io.stuff)
+}
+
+// Instantiate module in another clock domain with implicit reset.
+class MultiClockModule2 extends Module {
+  val io = IO(new Bundle {
+    val clockB = Input(Clock())
+    val stuff = Input(Bool())
+  })
+  val clockB_child = withClock(io.clockB) { Module(new ChildModule) }
+  clockB_child.io.in := io.stuff
+}
+
+class ChildModule extends Module {
+  val io = IO(new Bundle{
+    val in = Input(Bool())
+  })
+}
+
+```