lint: silence clippy::unnecessary_cast on glibc + fix doc list indent

CI's lint workflow runs clippy on linux target where:
- platform/fs_type/linux.rs and io/writeback/linux.rs: the i64 cast
  on buf.f_type / NFS_SUPER_MAGIC is unnecessary on glibc x86_64 (both
  already i64) but required on musl (c_ulong); silence the lint via
  inline allow with explanatory comment.
- mux/m2ts_mux/packet.rs: doc comment continuation across lines was
  parsed as an unindented list item. Reworded to a single flowing
  sentence.

src/io/writeback/linux.rs

@@ -321,8 +321,12 @@ fn detect_nfs(fd: RawFd) -> bool {
     }
     // `f_type` is signed (`__fsword_t`) on glibc and unsigned
     // (`c_ulong`) on musl. Cast both sides to i64 for a portable
-    // comparison.
+    // comparison. On glibc x86_64 both already are i64 — clippy flags
+    // the cast as unnecessary on that target only, but we need it for
+    // musl, so silence the lint.
+    #[allow(clippy::unnecessary_cast)]
     let f_type = buf.f_type as i64;
+    #[allow(clippy::unnecessary_cast)]
     let nfs_magic = libc::NFS_SUPER_MAGIC as i64;
     f_type == nfs_magic
 }