update dox for pathmeasure

Of note, the client need not keep the path around after creating/resetting the measure
object. The parts that are needed are copied, so the client is free to modify/delete
the path afterwards.

Bug: skia:
Change-Id: I9b9a0fd8e3cd6fd6c60de0017bcaee1fbc2fb893
Reviewed-on: https://skia-review.googlesource.com/c/190679
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
2 files changed