[docs] Remove distribution references from man page

We don't want to provide a history of what distributions
used before plymouth, because that history could get long,
isn't super relevant, and will add noise for a users looking
for help (they probably only care about their distributions,
not others).

docs/plymouth.8

@@ -8,13 +8,12 @@ plymouth \- A graphical boot system and logger
 .SH DESCRIPTION
 \fBplymouth\fP is a a graphical boot system for Linux which takes advantage of the kernel-based
 mode setting (KMS) available for modern graphic cards to provide a seamless, flickerfree
-and attractive boot screen. It supercedes the Red Hat graphical boot manager (RHGB) on Fedora
-and Red Hat systems and replaces usplash with xplash on Ubuntu and Debian. It allows
-to choose between various, static or animated graphical themes to spruce up the
-startup and avoid the noise generated by the vast amount of kernel messages while
-the machine boots into X. On systems where kernel-based mode setting is not available,
-plymouth falls back to a text mode boot screen which provides a simple progress bar
-to provide feedback during boot.
+and attractive boot screen. It allows to choose between various, static or
+animated graphical themes to spruce up the startup and avoid the noise
+generated by the vast amount of kernel messages while the machine boots into X.
+On systems where kernel-based mode setting is not available, plymouth falls
+back to a text mode boot screen which provides a simple progress bar to provide
+feedback during boot.
 .PP
 To configure plymouth, that is to choose and install the preferred boot theme, the
 user has to invoke \fBplymouth-set-default-theme\fP. It changes the configuration