document that libc.a can only be used when __ANDROID_API__ is the latest API level (but that does not affect compatibility, because -static)

bash can be successfully built with ndk r21, but can not be built with ndk r22-r23 due to following error:

```
ld: error: duplicate symbol: strchrnul
>>> defined at strchrnul.c
>>> strchrnul.o:(strchrnul) in archive ./lib/sh/libsh.a
>>> defined at static_function_dispatch.S:39 (bionic/libc/arch-arm64/static_function_dispatch.S:39)
>>> static_function_dispatch.o:(.text+0x30) in archive /home/kubast2/Android/Sdk/ndk/22.1.7171670/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/21/libc.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Makefile:582: bash] Error 1
```

from the error log, the `strchrnul` function exists in android-21's `libc.a`, but actually it was introduced in android-24

```
#if __ANDROID_API__ >= 24
char* strchrnul(const char* __s, int __ch) __attribute_pure__ __INTRODUCED_IN(24);
#endif /* __ANDROID_API__ >= 24 */
```

Start by reviewing the reported bash build failure with NDK r22-r23 and the referenced android-21 libc.a and static_function_dispatch.S paths. Document that libc.a is usable only with the latest __ANDROID_API__ level, and explain why static linking does not affect compatibility; done when the behavior and version range are clear to NDK users.

Written by the indexing model from the issue text.