docs: document that pmbr shouldn't be used unless nedded (#136)

While testing a new build of our image, we found that it didn't boot in
UEFI mode on a Lenovo M700 and tracked it down to this. I think it makes
sense to call out this caveat in the rustdoc.

src/lib.rs

@@ -1246,6 +1246,10 @@ impl GPT {
     /// MBR partition table is marked as bootable. Some legacy BIOS systems do not consider a disk
     /// to be bootable if there isn't an MBR partition marked as bootable in the MBR partition
     /// table.
+    ///
+    /// <div class="warning">Some systems will not consider a disk to be bootable in UEFI mode
+    /// if the pMBR is marked as bootable, so this should only be used if booting on legacy BIOS
+    /// systems is a requirement.</div>
     pub fn write_bootable_protective_mbr_into<W: ?Sized>(
         mut writer: &mut W,
         sector_size: u64,