unstable-book: Document -Z tls-model

Author: petrochenkov

src/doc/unstable-book/src/compiler-flags/tls-model.md

@@ -0,0 +1,25 @@
+# `tls_model`
+
+The tracking issue for this feature is: None.
+
+------------------------
+
+Option `-Z tls-model` controls [TLS model](https://www.akkadia.org/drepper/tls.pdf) used to
+generate code for accessing `#[thread_local]` `static` items.
+
+Supported values for this option are:
+
+- `global-dynamic` - General Dynamic TLS Model (alternatively called Global Dynamic) is the most
+general option usable in all circumstances, even if the TLS data is defined in a shared library
+loaded at runtime and is accessed from code outside of that library.  
+This is the default for most targets.
+- `local-dynamic` - model usable if the TLS data is only accessed from the shared library or
+executable it is defined in. The TLS data may be in a library loaded after startup (via `dlopen`).
+- `initial-exec` - model usable if the TLS data is defined in the executable or in a shared library
+loaded at program startup.
+The TLS data must not be in a library loaded after startup (via `dlopen`).
+- `local-exec` - model usable only if the TLS data is defined directly in the executable,
+but not in a shared library, and is accessed only from that executable.
+
+`rustc` and LLVM may use a more optimized model than specified if they know that we are producing
+and executable rather than a library, or that the `static` item is private enough.