docs: added note in animation keyframe offset (angular#32350)

Not mentioned in the docs.
Whenever offset property is used inside a keyframe's step at least once, then it must be defined to all the steps.
When read the first time, I supposed that the API automatically sets an even offset to the remaining not defined offsets, which is not

PR Close angular#32350

aio/content/guide/transition-and-triggers.md

@@ -227,7 +227,7 @@ The code for this color change might look like this.
 
 ### Offset
 
-Keyframes include an *offset* that defines the point in the animation where each style change occurs. Offsets are relative measures from zero to one, marking the beginning and end of the animation, respectively.
+Keyframes include an *offset* that defines the point in the animation where each style change occurs. Offsets are relative measures from zero to one, marking the beginning and end of the animation, respectively and should be applied to each of the keyframe's steps if used at least once.
 
 Defining offsets for keyframes is optional. If you omit them, evenly spaced offsets are automatically assigned. For example, three keyframes without predefined offsets receive offsets of 0, 0.5, and 1. Specifying an offset of 0.8 for the middle transition in the above example might look like this.