libtiff: Documentation update: TIFFSetTagExtender() cannot add tags to custom directories.

From 9f0c7bff9b3cc108455002db54fd52183bc649fd Mon Sep 17 00:00:00 2001
From: Su_Laus <[EMAIL REDACTED]>
Date: Tue, 16 May 2023 23:02:03 +0200
Subject: [PATCH] Documentation update: TIFFSetTagExtender() cannot add tags to
 custom directories.

Closes #557.
 doc/addingtags.rst                   | 20 +++++++++-----------
 doc/functions/TIFFSetTagExtender.rst |  6 ++++++
 doc/specification/coverage.rst       |  2 +-
 3 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/doc/addingtags.rst b/doc/addingtags.rst
index c035c75d..96f35a82 100644
--- a/doc/addingtags.rst
+++ b/doc/addingtags.rst
@@ -54,7 +54,7 @@ This structure looks like this:
     Generally the same as field_readcount.  A few built-in exceptions exist, but
     I haven't analysed why they differ.
-.. c:member:: TIFFDataType TIFFDataType.field_type
+.. c:member:: TIFFDataType TIFFFieldInfo.field_type
     Type of the field.  One of :c:enumerator:`TIFF_BYTE`, :c:enumerator:`TIFF_ASCII`,
     :c:enumerator:`TIFF_SHORT`, :c:enumerator:`TIFF_LONG`,
@@ -63,11 +63,10 @@ This structure looks like this:
     :c:enumerator:`TIFF_SLONG`, :c:enumerator:`TIFF_SRATIONAL`,
     :c:enumerator:`TIFF_FLOAT`, :c:enumerator:`TIFF_DOUBLE` or
-.. note::
-    Some fields can support more than one type (for
-    instance short and long).  These fields should have multiple TIFFFieldInfos.
+    And for BigTIFF :c:enumerator:`TIFF_LONG8`,
+    :c:enumerator:`TIFF_SLONG8` and :c:enumerator:`TIFF_IFD8`,
+    which are automatically reverted to 4 byte fields in
+    ClassicTIFF.
 .. c:member:: unsigned short TIFFFieldInfo.field_bit
@@ -95,15 +94,14 @@ This structure looks like this:
     A name for the tag.  Normally mixed case (studly caps)
     like ``StripByteCounts``, and relatively short.
-A :c:struct:`TIFFFieldInfo` definition exists for each built-in tag in the
-:file:`tif_dirinfo.c` file.  Some tags which support multiple data types
-have more than one definition, one per data type supported.
+Within :file:`tif_dirinfo.c` file, the built-in TIFF tags are defined with
+:c:struct:`TIFFField` structure that has additional parameters defining the var_arg
+interface of :c:func:`TIFFSetField` and :c:func:`TIFFGetField`.
 Various functions exist for getting the internal :c:struct:`TIFFFieldInfo`
 definitions, including :c:func:`_TIFFFindFieldInfo`, and
 :c:func:`_TIFFFindFieldInfoByName`.  See
-:file:`tif_dirinfo.c` for details.  There must be some mechanism to get the whole
-list, though I don't see it off hand.
+:file:`tif_dirinfo.c` for details.
 .. _Tag_Auto_registration:
diff --git a/doc/functions/TIFFSetTagExtender.rst b/doc/functions/TIFFSetTagExtender.rst
index 001ec1f6..ea72cbbe 100644
--- a/doc/functions/TIFFSetTagExtender.rst
+++ b/doc/functions/TIFFSetTagExtender.rst
@@ -19,6 +19,12 @@ Description
 for user defined tags as an extender callback with ``libtiff``.
 A brief description can be found at :ref:`define_application_tags`.
+.. note::
+    :c:func:`TIFFSetTagExtender` cannot be used to extend custom directories
+    (for example EXIF, GPS or other custom directories).
+    Only the TIFF directory can be extended with additional tags.
 See also
diff --git a/doc/specification/coverage.rst b/doc/specification/coverage.rst
index 75bc7c5f..9f4bf7db 100644
--- a/doc/specification/coverage.rst
+++ b/doc/specification/coverage.rst
@@ -1466,7 +1466,7 @@ pre-multipled by Alpha.  If this means nothing to you, check out
 Porter & Duff's paper in the '84 SIGGRAPH proceedings: "Compositing Digital
-Tag ``RichTIFFIPTC`` (33723)is defined wrongly in TIFF/EP definition as "LONG or ASCII".
+Tag ``RichTIFFIPTC`` (33723) is defined wrongly in TIFF/EP definition as "LONG or ASCII".
 ``libtiff`` defines it as "UNDEFINED or BYTE".
 The ``ImageDepth``