filesystem: Note initramfs reneneration is required for prepare-root

Ref https://gitlab.com/fedora/bootc/examples/-/merge_requests/59#note_2500726902

Author: cgwalters

docs/src/filesystem.md

@@ -211,6 +211,8 @@ This can be combined with `etc.transient` as well (below).
 
 More on prepare-root: <https://ostreedev.github.io/ostree/man/ostree-prepare-root.html>
 
+Note that regenerating the initramfs is required when changing this file.
+
 ## Enabling transient etc
 
 The default (per above) is to have `/etc` persist. If however you do
@@ -229,6 +231,8 @@ This can be combined with `root.transient` as well (above).
 
 More on prepare-root: <https://ostreedev.github.io/ostree/man/ostree-prepare-root.html>
 
+Note that regenerating the initramfs is required when changing this file.
+
 ## Enabling state overlays
 
 This feature enables a writable overlay on top of `/opt` (or really, any