Add note about "Conflicting distribution" warning to older releases page

Fixes mono/docker#73

docs/getting-started/install/linux/index.md

@@ -14,6 +14,10 @@ Accessing older releases
 
 If for some reason you want to pin an older version of Mono rather than updating to the latest, you can modify the Debian repository to "stable-jessie/snapshots/X.XX.X" instead of "stable-jessie". For example, "stable-jessie/snapshots/3.10.0" will lock you to that version.
 
+_Note:_ you'll get a warning similar to this one when running `apt update`, it is harmless and can be ignored:
+
+> W: Conflicting distribution: https://download.mono-project.com stable-jessie/snapshots/3.10.0.160 InRelease (expected stable-jessie/snapshots but got jessie)
+
 These snapshots will cease receiving updates as soon as the next major Mono version gets uploaded - for example, as soon as Mono 3.12 gets uploaded, 3.10 will never receive updates.
 
 On RPM distributions, force the package version in your package manager - all older versions are published in the YUM metadata and should be available.