Clarify the case sensitivity of .editorconfig file

This has been generally assumed since we don't specifically mention other
capitalizations, and core libraries have also assumed so. However, some
communities seem to have used capital letters in some places, and it's best if
we can clarify.

If we go the other route, as a standard, the current core libraries would break
because they don't work consistently with each other. Additionally, core
libraries are cumbersome to implement on case-sensitive filesystems, because
they always have to use some regex to search the `.editorconfig` file in a
directory, instead of just pointing to the file named `.editorconfig`.

Close editorconfig/editorconfig#523

Author: xuhdev

index.rst

@@ -185,7 +185,7 @@ If the glob contains a path separator (a ``/`` not inside square brackets), then
 to the directory level of the particular `.editorconfig` file itself.
 Otherwise the pattern may also match at any level below the `.editorconfig`
 level. For example, ``*.c`` matches any file that ends with ``.c`` in the
-directory of ``.editorconfig`` or any other directory below one that stores this ``.editorconfig``. 
+directory of ``.editorconfig`` or any other directory below one that stores this ``.editorconfig``.
 However, the glob ``subdir/*.c`` only matches files that end
 with ``.c`` in the ``subdir`` directory in the directory of ``.editorconfig``.
 
@@ -202,18 +202,32 @@ File Processing
 
 When a filename is given to EditorConfig a search is performed in the
 directory of the given file and all parent directories for an EditorConfig
-file (named ".editorconfig" by default). Non-existing directories are treated
-as if they exist and are empty. All found EditorConfig files are
-searched for sections with section names matching the given filename. The
-search shall stop if an EditorConfig file is found with the ``root``
-key set to ``true`` in the preamble or when reaching the root
+file. An EditorConfig file is named ".editorconfig", all lowercased.
+Non-existing directories are treated as if they exist and are empty. All found
+EditorConfig files are searched for sections with section names matching the
+given filename. The search shall stop if an EditorConfig file is found with
+the ``root`` key set to ``true`` in the preamble or when reaching the root
 filesystem directory.
 
 Files are read top to bottom and the most recent rules found take
 precedence. If multiple EditorConfig files have matching sections, the rules
 from the closer EditorConfig file are read last, so pairs in closer
 files take precedence.
 
+Capitalization of the File Name
+-------------------------------
+
+The ``.editorconfig`` filename should be lowercased. On some platforms,
+opening a file with a different capitalization results in opening the same
+file with lowercased file names. This is common with a case-insensitive
+filesystem. For example, a file named ``.editorConfig`` exists in the
+filesystem, but opening a file named ``.editorconfig`` via a file-opening API
+still opens the differently capitalized ``.editorConfig`` file.
+
+On such a platform, a Core may choose to treat the file with a different
+capitalization as if it were all lowercased. This is to prevent the need of
+retrieving the file name after opening it.
+
 .. _supported-pairs:
 
 Supported Pairs