added notes about converting types on columns other than _value

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/tobool.md

@@ -19,6 +19,12 @@ _**Output data type:** Boolean_
 toBool()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `bool()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/toduration.md

@@ -19,6 +19,12 @@ _**Output data type:** Duration_
 toDuration()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `duration()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/tofloat.md

@@ -19,6 +19,12 @@ _**Output data type:** Float_
 toFloat()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `float()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/toint.md

@@ -19,6 +19,12 @@ _**Output data type:** Integer_
 toInt()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `int()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/tostring.md

@@ -19,6 +19,12 @@ _**Output data type:** String_
 toString()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `string()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/totime.md

@@ -19,6 +19,12 @@ _**Output data type:** Time_
 toTime()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `time()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")

content/v2.0/reference/flux/functions/built-in/transformations/type-conversions/touint.md

@@ -19,6 +19,12 @@ _**Output data type:** UInteger_
 toUInt()
 ```
 
+{{% note %}}
+To convert values in a column other than `_value`, define a custom function
+patterned after the [function definition](#function-definition),
+but replace the column in the `uint()` function with your desired column.
+{{% /note %}}
+
 ## Examples
 ```js
 from(bucket: "telegraf")