Compiler Attributes: improve explanation of header

Explain better what "optional" attributes are, and avoid calling
them so to avoid confusion. Simply retain "Optional" as a word
to look for in the comments.

Moreover, add a couple sentences to explain a bit more the intention
and the documentation links.

Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

include/linux/compiler_attributes.h

@@ -4,22 +4,26 @@
 
 /*
  * The attributes in this file are unconditionally defined and they directly
- * map to compiler attribute(s) -- except those that are optional.
+ * map to compiler attribute(s), unless one of the compilers does not support
+ * the attribute. In that case, __has_attribute is used to check for support
+ * and the reason is stated in its comment ("Optional: ...").
  *
  * Any other "attributes" (i.e. those that depend on a configuration option,
  * on a compiler, on an architecture, on plugins, on other attributes...)
  * should be defined elsewhere (e.g. compiler_types.h or compiler-*.h).
+ * The intention is to keep this file as simple as possible, as well as
+ * compiler- and version-agnostic (e.g. avoiding GCC_VERSION checks).
  *
  * This file is meant to be sorted (by actual attribute name,
  * not by #define identifier). Use the __attribute__((__name__)) syntax
  * (i.e. with underscores) to avoid future collisions with other macros.
- * If an attribute is optional, state the reason in the comment.
+ * Provide links to the documentation of each supported compiler, if it exists.
  */
 
 /*
- * To check for optional attributes, we use __has_attribute, which is supported
- * on gcc >= 5, clang >= 2.9 and icc >= 17. In the meantime, to support
- * 4.6 <= gcc < 5, we implement __has_attribute by hand.
+ * __has_attribute is supported on gcc >= 5, clang >= 2.9 and icc >= 17.
+ * In the meantime, to support 4.6 <= gcc < 5, we implement __has_attribute
+ * by hand.
  *
  * sparse does not support __has_attribute (yet) and defines __GNUC_MINOR__
  * depending on the compiler used to build it; however, these attributes have