docs/qapi-code-gen.txt: Clarify naming rules

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1488317230-26248-24-git-send-email-armbru@redhat.com>

docs/qapi-code-gen.txt

@@ -216,33 +216,38 @@ single-dimension array of that type; multi-dimension arrays are not
 directly supported (although an array of a complex struct that
 contains an array member is possible).
 
+All names must begin with a letter, and contain only ASCII letters,
+digits, hyphen, and underscore.  There are two exceptions: enum values
+may start with a digit, and names that are downstream extensions (see
+section Downstream extensions) start with underscore.
+
+Names beginning with 'q_' are reserved for the generator, which uses
+them for munging QMP names that resemble C keywords or other
+problematic strings.  For example, a member named "default" in qapi
+becomes "q_default" in the generated C code.
+
 Types, commands, and events share a common namespace.  Therefore,
 generally speaking, type definitions should always use CamelCase for
-user-defined type names, while built-in types are lowercase. Type
-definitions should not end in 'Kind', as this namespace is used for
-creating implicit C enums for visiting union types, or in 'List', as
-this namespace is used for creating array types.  Command names,
-and member names within a type, should be all lower case with words
-separated by a hyphen.  However, some existing older commands and
-complex types use underscore; when extending such expressions,
-consistency is preferred over blindly avoiding underscore.  Event
-names should be ALL_CAPS with words separated by underscore.  Member
-names cannot start with 'has-' or 'has_', as this is reserved for
-tracking optional members.
+user-defined type names, while built-in types are lowercase.
+
+Type names ending with 'Kind' or 'List' are reserved for the
+generator, which uses them for implicit union enums and array types,
+respectively.
+
+Command names, and member names within a type, should be all lower
+case with words separated by a hyphen.  However, some existing older
+commands and complex types use underscore; when extending such
+expressions, consistency is preferred over blindly avoiding
+underscore.
+
+Event names should be ALL_CAPS with words separated by underscore.
+
+Member names starting with 'has-' or 'has_' are reserved for the
+generator, which uses them for tracking optional members.
 
 Any name (command, event, type, member, or enum value) beginning with
 "x-" is marked experimental, and may be withdrawn or changed
-incompatibly in a future release.  All names must begin with a letter,
-and contain only ASCII letters, digits, dash, and underscore.  There
-are two exceptions: enum values may start with a digit, and any
-extensions added by downstream vendors should start with a prefix
-matching "__RFQDN_" (for the reverse-fully-qualified-domain-name of
-the vendor), even if the rest of the name uses dash (example:
-__com.redhat_drive-mirror).  Names beginning with 'q_' are reserved
-for the generator: QMP names that resemble C keywords or other
-problematic strings will be munged in C to use this prefix.  For
-example, a member named "default" in qapi becomes "q_default" in the
-generated C code.
+incompatibly in a future release.
 
 In the rest of this document, usage lines are given for each
 expression type, with literal strings written in lower case and
@@ -643,6 +648,18 @@ any non-empty complex type (struct, union, or alternate), and a
 pointer to that QAPI type is passed as a single argument.
 
 
+=== Downstream extensions ===
+
+QAPI schema names that are externally visible, say in the Client JSON
+Protocol, need to be managed with care.  Names starting with a
+downstream prefix of the form __RFQDN_ are reserved for the downstream
+who controls the valid, reverse fully qualified domain name RFQDN.
+RFQDN may only contain ASCII letters, digits, hyphen and period.
+
+Example: Red Hat, Inc. controls redhat.com, and may therefore add a
+downstream command __com.redhat_drive-mirror.
+
+
 == Client JSON Protocol introspection ==
 
 Clients of a Client JSON Protocol commonly need to figure out what