sw: document SwInputField and SwTextInputField

To make it more obvious how SwTextInputField owns an SwFormatField,
which then further owns an SwInputField.

Change-Id: I1b5e020e45a14eecff72b03ee63bb1ad1d55cfc4
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194206
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

sw/inc/expfld.hxx

@@ -285,6 +285,10 @@ public:
     SwDoc* GetDoc() const { return mpDoc; }
 };
 
+/// SwField subclass for input fields, which are editable in-place. Typically owned by an
+/// SwFormatField, which is then further owned by SwTextInputField to track its location.
+///
+/// Insert -> Field -> More Fields -> Functions -> Input field on the UI.
 class SW_DLLPUBLIC SwInputField final : public SwField
 {
     mutable OUString maContent;

sw/inc/txtfld.hxx

@@ -76,6 +76,8 @@ public:
 
 };
 
+/// SwTextAttr subclass that tracks the location of the wrapped SwFormatField, when the format
+/// field's field is an SwInputField.
 class SwTextInputField final
     : public SwTextAttrNesting
     , public SwTextField