Document that the spec uses RFC 2119 (#1928)

Signed-off-by: HarHarLinks <2803622+HarHarLinks@users.noreply.github.com>

changelogs/appendices/newsfragments/1928.clarification

@@ -0,0 +1 @@
+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/application_service/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/client_server/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/identity_service/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/internal/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/push_gateway/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/room_versions/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

changelogs/server_server/newsfragments/1928.clarification

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+Document that the spec uses RFC 2119. Contributed by @HarHarLinks.

content/_index.md

@@ -99,6 +99,20 @@ services - be that for instant messages, VoIP call setups, or any other
 objects that need to be reliably and persistently pushed from A to B in
 an interoperable and federated manner.
 
+### Requirement levels
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" across all parts of the
+specification are to be interpreted as described in
+[RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119).
+
+Some entire sections of the specification might be optional depending on the
+circumstances. For example, the
+[modules of the client-server API](/client-server-api/#modules)
+apply depending on the use case. The requirement level expressed by the above
+key words appearing in such a section is only relevant if the section itself is
+applicable.
+
 ### Spec Change Proposals
 
 To propose a change to the Matrix Spec, see the explanations at