[PATCH b4 v2] docs: document --set-presubject and b4.send-presubject

[Antonin Godard <antonin.godard@bootlin.com>]

Document the b4 prep --set-presubject option and b4.send-presubject
setting, added by commit d7232f5db2e4 ("Add support for setting a
presubject").

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
---
Changes in v2:
- Rebase on master and fix the conflicts
- Link to v1: https://patch.msgid.link/20260121-docs-presubject-v1-1-d625a279f1c5@bootlin.com
---
 docs/config.rst           | 13 +++++++++++++
 docs/contributor/prep.rst | 23 +++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/docs/config.rst b/docs/config.rst
index d2a80c2..fd4ca3e 100644
--- a/docs/config.rst
+++ b/docs/config.rst
@@ -485,6 +485,19 @@ Contributor-oriented settings
 
      .. versionadded:: v0.11
 
+   :term:`b4.send-presubject`
+     Identifiers to include before ``[PATCH]``. These **must** be enclosed
+     with brackets. For example::
+
+         [b4]
+         send-presubject = "[myproject]"
+
+     This setting can be replaced for a series with ``b4 prep --set-presubject``.
+
+     Default: ``None``
+
+     .. versionadded:: v0.15
+
    :term:`b4.send-same-thread`
      When sending a new version of a series, send it in the same thread as
      the previous version. The config supports the following values:
diff --git a/docs/contributor/prep.rst b/docs/contributor/prep.rst
index e6b45c7..1ec119b 100644
--- a/docs/contributor/prep.rst
+++ b/docs/contributor/prep.rst
@@ -490,6 +490,29 @@ modifying defaults for some of these flags.
 
   .. versionadded:: v0.13
 
+``--set-presubject [PRESUBJECT]``
+  If you want to include a project name or any other identifier before the
+  subject of your patch, you can define them via this command. These are
+  placed **before** ``[PATCH ...]`` in the subject, and **must** be
+  enclosed with brackets.
+
+  For example, running the following command::
+
+      b4 prep --set-presubject "[myproject]"
+
+  Will render your patch(es) as follows::
+
+      [myproject][PATCH ...] <title>
+
+  These do not collide with prefixes set with ``--set-prefixes`` or
+  ``--add-prefixes`` or patch versions, which are located next to ``PATCH``
+  in the subject.
+
+  To remove any presubject you previously set, you can run ``b4 prep
+  --set-presubject ''``.
+
+  .. versionadded:: v0.15
+
 ``--cleanup [BRANCHNAME]``
   Archive and delete obsolete prep-managed branches and all git objects
   related to them, such as sent tags. Run without parameters to list

---
base-commit: 858baa2b2eeb93ea1b0e1b57adfb77e4a126043e
change-id: 20260121-docs-presubject-0a5e36c74b5b