clarify NewContext's inheritance and cancellation

We have been developing this behavior over the past few weeks, but it
wasn't properly documented anywhere.

Fixes #303.

allocate.go

@@ -23,8 +23,8 @@ type Allocator interface {
 	Allocate(context.Context, ...BrowserOption) (*Browser, error)
 
 	// Wait blocks until an allocator has freed all of its resources.
-	// Cancelling the context obtained via NewAllocator will already perform
+	// Cancelling the allocator context will already perform this operation,
-	// this operation, so normally there's no need to call Wait directly.
+	// so normally there's no need to call Wait directly.
 	Wait()
 }
 
@@ -52,7 +52,7 @@ var DefaultExecAllocatorOptions = []ExecAllocatorOption{
 }
 
 // NewExecAllocator creates a new context set up with an ExecAllocator, suitable
-// for use with NewContext or Run.
+// for use with NewContext.
 func NewExecAllocator(parent context.Context, opts ...ExecAllocatorOption) (context.Context, context.CancelFunc) {
 	ctx, cancel := context.WithCancel(parent)
 	c := &Context{Allocator: setupExecAllocator(opts...)}

chromedp.go

@@ -62,7 +62,17 @@ type Context struct {
 	cancelErr error
 }
 
-// NewContext creates a browser context using the parent context.
+// NewContext creates a chromedp context from the parent context. The parent
+// context's Allocator is inherited, defaulting to an ExecAllocator with
+// DefaultExecAllocatorOptions.
+//
+// If the parent context contains an allocated Browser, the child context
+// inherits it, and its first Run creates a new tab on that browser. Otherwise,
+// its first Run will allocate a new browser.
+//
+// Cancelling the returned context will close a tab or an entire browser,
+// depending on the logic described above. To cancel a context while checking
+// for errors, see Cancel.
 func NewContext(parent context.Context, opts ...ContextOption) (context.Context, context.CancelFunc) {
 	ctx, cancel := context.WithCancel(parent)