[3.15] gh-154060: Fix the Profile Stats offsets in the profiling binary format docs and a missing flag (GH-157871) (#158855)

gh-154060: Fix the `Profile Stats` offsets in the profiling binary format docs and a missing flag (GH-157871)

* correct profile stats size and paddings

* missing flag

* gh-154060: Clarify the main-thread status flag

---------
(cherry picked from commit 8df2580)

Co-authored-by: Maurycy Pawłowski-Wieroński <maurycy@maurycy.com>
Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: 3 people

InternalDocs/profiling_binary_format.md

@@ -47,8 +47,8 @@ The file consists of five required sections and one optional extension:
 |   String Table   |  Variable size
 +------------------+  frame_table_offset
 |   Frame Table    |  Variable size
-+------------------+  file_size - 64 (when stats are present)
-| Profile Stats    |  32 bytes (optional)
++------------------+  file_size - 88 (when stats are present)
+| Profile Stats    |  56 bytes (optional)
 +------------------+  file_size - 32
 |     Footer       |  32 bytes (fixed)
 +------------------+  file_size
@@ -220,6 +220,7 @@ The status byte is a bitfield encoding thread state at sample time:
 |  2  | THREAD_STATUS_UNKNOWN | Thread state could not be determined       |
 |  3  | THREAD_STATUS_GIL_REQUESTED | Thread is waiting to acquire the GIL  |
 |  4  | THREAD_STATUS_HAS_EXCEPTION | Thread has a pending exception         |
+|  5  | THREAD_STATUS_MAIN_THREAD | Thread is the interpreter's main thread |
 
 Multiple flags can be set simultaneously (e.g., a thread can hold the GIL
 while also running on CPU). Analysis tools use these to filter samples or