Android: Clarify DFM docs for updates

This came up in a mailing list discussion:
http://g/sampling-profiler-core/lzZlIlY5PLM/xLWKG5PEAQAJ

Once a DFM is installed by Play, it will be updated alongside Chrome. A
separate reinstall will not be necessary on every app update.

Bug: None
Change-Id: I7d839b2161e72cb81c77796336918ede1b0e7f75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1963094
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723998}

docs/android_dynamic_feature_modules.md

@@ -715,7 +715,9 @@ $ $OUTDIR/bin/monochrome_public_bundle launch --args="--fake-feature-module-inst
 When running the install code, the Foo DFM module will be emulated.
 This will be the case in production right after installing the module. Emulation
 will last until Play Store has a chance to install your module as a true split.
-This usually takes about a day.
+This usually takes about a day. After it has been installed, it will be updated
+atomically alongside Chrome. Always check that it is installed and available
+before invoking code within the DFM.
 
 *** note
 **Warning:** There are subtle differences between emulating a module and