tentatively remove the "B" RISC-V extension from the documentation

Although the "B" extension is redefined and ratified, keeping this in the
documentation as-is have two issues:

*   "B" extension is not added to `riscv.rs` yet (to be added later).
*   "B" extension is ratified as a combination of "Zba", "Zbb" and "Zbs"
    extensions and "Zbc" is *not* a part of "B" itself (despite that
    it is listed under "B"), which makes the documentation misleading.

This commit tentatively removes the reference to the "B" extension and
replaced with "Bit Manipulation Extensions" without an extension name.

library/stdarch/crates/std_detect/src/detect/arch/riscv.rs

@@ -28,7 +28,7 @@ features! {
     /// * RV32I: `"rv32i"`
     /// * RV64I: `"rv64i"`
     /// * A: `"a"`
-    /// * B: `"b"`
+    /// * Bit-Manipulation Extensions:
     ///   * Zba: `"zba"`
     ///   * Zbb: `"zbb"`
     ///   * Zbc: `"zbc"`