trivial: tighten deprecation warnings

- Document the warning convention for deprecated features in chapter 1.
- Use the convention for the warning about the deprecated spin table
  protocol in chapter 3.
- Tighten a couple of warnings in chapter 4 to spare lines.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

source/chapter1-about.rst

@@ -172,6 +172,12 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
 "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
 interpreted as described in :rfc:`2119`.
 
+Features, which will not be supported by a future version of this specification
+are indicated with a warning such as the following one:
+
+.. warning:: This feature is deprecated.
+   A future version of this specification will disallow its use.
+
 Typographic conventions
 -----------------------
 

source/chapter3-secureworld.rst

@@ -25,9 +25,9 @@ Platforms without EL3 must implement one of:
 - PSCI and SMCCC at EL2 (leaving only EL1 available to an operating system)
 - Linux AArch64 spin tables [LINUXA64BOOT]_ (Devicetree only)
 
-However, the spin table protocol is strongly discouraged.
-Future versions of this specification will only allow PSCI and SMCCC, and they
-should be implemented in all new designs.
+.. warning:: The spin table protocol is strongly discouraged.
+   Future versions of this specification will only allow PSCI and SMCCC, and
+   they should be implemented in all new designs.
 
 It is recommended that firmware implementing PSCI supports version 1.0 or later
 [#PSCINote]_ and that firmware implementing SMCCC supports version 1.1 or later

source/chapter4-firmware-media.rst

@@ -61,9 +61,8 @@ In which case, a legacy Master Boot Record (MBR) must be used.
    the GPT disk layout.
    In both cases the shared storage device must use legacy MBR partitioning.
 
-.. warning::
-
-   MBR partitioning is deprecated and only included for legacy support.
+.. warning:: MBR partitioning is deprecated and only included for legacy
+   support.
    All new platforms are expected to use GPT partitioning.
    GPT partitioning supports a much larger number of partitions, and
    has built in resiliency.
@@ -87,9 +86,8 @@ delete, move, or modify protective partition entries.
 Manual partitioning tools should provide warnings when modifying
 protective partitions.
 
-.. warning::
-
-   Fixed offsets to firmware data is supported only for legacy reasons.
+.. warning:: Fixed offsets to firmware data is supported only for legacy
+   reasons.
    All new platforms are expected to use partitions to locate firmware files.
 
    A future version of this specification will disallow the use of fixed