docs: add version-history notes for fixed start_pos and error positions

Two bugs of 3.12.0 are fixed on develop without a note on the pages.
With JSON_DIAGNOSTIC_POSITIONS, start_pos() of a string that contains
escape sequences was too large; the position is now that of the opening
quote. A parse error for an unexpected number that is followed by a line
feed reported column 0 (for "1 2" and a line feed, line 1, column 0); for
"1", line feed, "2", line feed it now reports line 2, column 1.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/parse.md

@@ -313,6 +313,10 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
   it as end of input; planned to become the default in version 4.0.0.
 - Extended empty-input detection to also cover an `std::istream` without a stream buffer, and fixed a crash
   (`std::terminate`) when parsing from an `std::istream` with `eofbit` in its exception mask, in version 3.13.0.
+- Fixed the position that a [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) reports for an
+  unexpected number that is followed by a line feed in version 3.13.0: for the input `1`, line feed, `2`, line feed,
+  version 3.12.0 reported "line 2, column 0" and version 3.13.0 reports "line 2, column 1"; for `1 2` and a line feed,
+  the report was "line 1, column 0" and is now "line 1, column 3".
 - Parsing from an `std::istream` scans the characters in its stream buffer in bulk in version 3.13.0
   ([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) restores reading one character at a
   time).

docs/mkdocs/docs/api/basic_json/start_pos.md

@@ -67,3 +67,7 @@ Constant.
 ## Version history
 
 - Added in version 3.12.0.
+- Fixed in version 3.13.0: in version 3.12.0, the start position of a string that contains escape sequences (for
+  example `#!cpp "\n"` or `#!cpp "\u00e4"`) was too large, by the number of bytes the escape sequences add to the
+  source text. The position is now always that of the opening `"`. The "(bytes a-b)" range in the messages of
+  [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) was wrong in the same way.

docs/mkdocs/docs/api/macros/json_diagnostic_positions.md

@@ -125,3 +125,5 @@ When the macro is not defined, the library will define it to its default value.
 ## Version history
 
 - Added in version 3.12.0.
+- Fixed in version 3.13.0: in version 3.12.0, [`start_pos()`](../basic_json/start_pos.md) of a string that contains
+  escape sequences was too large; it is now the position of the opening `"`.