doc: sframe: small improvements for readability

Update some of the content to make the specification document hopefully
clearer:
  - Fix some typos.
  - Use Title case consistently for headings.
  - Update text around detection of foreign endianness.
  - Split the structure field "Name" in each table to two separate
    colunms for additional attention: "Type" and "Name".
  - Rename "SFrame endianness" section to "SFrame magic number and
    endianness"
  - Update text around provisions for extending SFrame for future
    ABIs/architectures.  Make it clear by tagging all provisions with an
    explicit index item "Provisions for future ABIs".
  - Add a paragraph on sort order of SFrame FDEs.
  - Add a statement for SFRAME_F_FRAME_POINTER flag.
  - Add a statement to assert that SFrame version 1 is now obsolete and
    should not be used.

libsframe/
	* doc/sframe-spec.texi: Small improvements for readability.