message: Clarify the documentation text, and include a common example.
diff: @@ -67,7 +67,33 @@ logger, with `WARNING` severity. To direct these warnings to `stdout`, use
  This exception is raised whenever a `tf.Tensor` is type-cast as a Python `bool`,
  in a context where eager execution is not active. The exception is only raised
  when graph execution is active, for example inside a `@tf.function` with
  -AutoGraph turned off. It can be caused by using a `tf.Tensor` value as:
  +AutoGraph turned off.
  +
  +**When AutoGraph is on**, it can be caused by:
  + * placing a Tensor-dependent `break`, `continue` or `return` inside a Python
  + loop (see example below)
  + * attempting to use a `tf.Tensor` in a list comprehension, by iterating over
  + it or using it in a condition)
  +
  +A typical example of mixing Python and TF control flow in an incompatible way
  +is:
  +
  +```
  +for i in range(3): # Python loop
  + if i > tf.constant(0): # TF conditional
  + break # raises OperatorNotAllowedInGraphError
  +```
  +
  +The way these errors are typically fixed is by ensuring all control flow is
  +TF control flow:
  +
  +```
  +for i in tf.range(3): # TF loop
  + if i > tf.constant(0): # TF conditional
  + break # works
  +```
  +
  +**When AutoGraph is off**, it can be caused by using a `tf.Tensor` value as:
  * the condition of an `if` or `while` statement: `if <tensor>:`
  * the argument in a logical expression: `tensor and another_tensor`
  @@ -75,9 +101,6 @@ AutoGraph turned off. It can be caused by using a `tf.Tensor` value as:
  Note: These operations are allowed when executing eagerly.
  -Within the context of AutoGraph, it usually indicates eager-style control
  -flow that has not been converted by AutoGraph, for any reason.
  -
  When encountering this error, make sure that the function is either decorated
  with `@tf.function`, or called from another function decorated in this way. Also
  look at the console and logging output for conversion warnings (see the section
  