Add various basic docstrings

src/c_str.rs

src/c_vec.rs

src/header/mod.rs

src/ld_so/mod.rs

src/lib.rs

src/platform/linux/mod.rs

src/platform/mod.rs

src/platform/pal/mod.rs

src/platform/redox/mod.rs

src/platform/types.rs

src/start.rs

src/sync/mod.rs