linux_l4re: annotate unstable *NUM constants

Some symbols are not stable. New upstream releases may change them. This
works out badly with SemVer. Advising users is necessary.

Author: dybucc

src/unix/linux_like/linux_l4re_shared.rs

@@ -683,6 +683,9 @@ pub const EI_CLASS: usize = 4;
 pub const ELFCLASSNONE: u8 = 0;
 pub const ELFCLASS32: u8 = 1;
 pub const ELFCLASS64: u8 = 2;
+
+/// This symbol is prone to change across releases upstream.
+/// See the [usage guidelines](crate#usage-guidelines) for details.
 pub const ELFCLASSNUM: usize = 3;
 
 pub const EI_DATA: usize = 5;
@@ -812,6 +815,9 @@ pub const EM_ALPHA: u16 = 0x9026;
 // elf.h - Legal values for e_version (version).
 pub const EV_NONE: u32 = 0;
 pub const EV_CURRENT: u32 = 1;
+
+/// This symbol is prone to change across releases upstream.
+/// See the [usage guidelines](crate#usage-guidelines) for details.
 pub const EV_NUM: u32 = 2;
 
 // elf.h - Legal values for p_type (segment type).