Rename text documentation from *.doc to *.txt

Rename plaintext documentation from doc/*.doc to doc/*.txt, to avoid
the chronic problem of web sites thinking they're in MS-Word format.
Sigh.  As if Microsoft somehow had a monopoly on the word "document".

diff --git a/doc/comboot.doc b/doc/comboot.txt
similarity index 100%
rename from doc/comboot.doc
rename to doc/comboot.txt

diff --git a/doc/distrib.doc b/doc/distrib.txt
similarity index 100%
rename from doc/distrib.doc
rename to doc/distrib.txt

diff --git a/doc/extlinux.doc b/doc/extlinux.txt
similarity index 100%
rename from doc/extlinux.doc
rename to doc/extlinux.txt

diff --git a/doc/isolinux.doc b/doc/isolinux.txt
similarity index 100%
rename from doc/isolinux.doc
rename to doc/isolinux.txt

diff --git a/doc/keytab-lilo.doc b/doc/keytab-lilo.txt
similarity index 100%
rename from doc/keytab-lilo.doc
rename to doc/keytab-lilo.txt

diff --git a/doc/memdisk.doc b/doc/memdisk.txt
similarity index 100%
rename from doc/memdisk.doc
rename to doc/memdisk.txt

diff --git a/doc/menu.doc b/doc/menu.txt
similarity index 100%
rename from doc/menu.doc
rename to doc/menu.txt

diff --git a/doc/pxelinux.doc b/doc/pxelinux.txt
similarity index 100%
rename from doc/pxelinux.doc
rename to doc/pxelinux.txt