Add some basic docs for unsafe attrs

Author: carbotaniuman

src/abi.md

@@ -66,12 +66,19 @@ item's name.
 Additionally, the item will be publicly exported from the produced library or
 object file, similar to the [`used` attribute](#the-used-attribute).
 
+This attribute is unsafe as an unmangled symbol may collide with another
+symbol with the same name (or a well-known symbol), leading to undefined behavior.
+
 ## The `link_section` attribute
 
 The *`link_section` attribute* specifies the section of the object file that a
 [function] or [static]'s content will be placed into. It uses the
 [_MetaNameValueStr_] syntax to specify the section name.
 
+This attribute is unsafe as using `link_section` allows users to place data
+and code into sections of memory not expecting them, such as mutable data
+into read-only areas.
+
 <!-- no_run: don't link. The format of the section name is platform-specific. -->
 ```rust,no_run
 #[no_mangle]
@@ -85,6 +92,9 @@ The *`export_name` attribute* specifies the name of the symbol that will be
 exported on a [function] or [static]. It uses the [_MetaNameValueStr_] syntax
 to specify the symbol name.
 
+This attribute is unsafe as a symbol with a custom name may collide with another
+symbol with the same name (or a well-known symbol), leading to undefined behavior.
+
 ```rust
 #[export_name = "exported_symbol_name"]
 pub fn name_in_rust() { }

src/attributes.md

@@ -4,9 +4,13 @@
 > **<sup>Syntax</sup>**\
 > _InnerAttribute_ :\
 > &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`
+> &nbsp;&nbsp; `#` `!` `[` `unsafe` `(` _Attr_ `)` `]`
 >
 > _OuterAttribute_ :\
 > &nbsp;&nbsp; `#` `[` _Attr_ `]`
+> &nbsp;&nbsp; `#` `[` `unsafe` `(` _Attr_ `)` `]`
+> &nbsp;&nbsp; `#` `!` `[` _Attr_ `]`
+> &nbsp;&nbsp; `#` `!` `[` `unsafe` `(` _Attr_ `)` `]`
 >
 > _Attr_ :\
 > &nbsp;&nbsp; [_SimplePath_] _AttrInput_<sup>?</sup>
@@ -29,6 +33,16 @@ Attributes other than macro attributes also allow the input to be an equals
 sign (`=`) followed by an expression. See the [meta item
 syntax](#meta-item-attribute-syntax) below for more details.
 
+An attribute may be considered unsafe. Unsafe attributes must *satisfy* extra
+safety conditions that cannot be checked by the compiler, and are specified
+as the attribute wrapped in `unsafe(...)`, for instance, `#[unsafe(no_mangle)]`.
+
+The following attributes are unsafe:
+
+* [`export_name`]
+* [`link_section`]
+* [`no_mangle`]
+
 Attributes can be classified into the following kinds:
 
 * [Built-in attributes]