Some documentation corrections at xml.h

[ThanasisMattas]

1st commit: Some documentation corrections at xml.h

2nd commit: Rearrangement of methods to aid reading xml.cpp parallel to xml.h .

[codecov]

Codecov Report

Merging #636 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #636      +/-   ##
==========================================
- Coverage   68.54%   68.53%   -0.01%     
==========================================
  Files         363      363              
  Lines       39978    39978              
==========================================
- Hits        27401    27400       -1     
- Misses      12577    12578       +1

Impacted Files	Coverage Δ
include/cantera/base/xml.h	100% <ø> (ø)	⬆️
src/base/xml.cpp	72.8% <100%> (ø)	⬆️
src/transport/GasTransport.cpp	90.77% <0%> (-0.21%)	⬇️

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[codecov]

Codecov Report

Merging #636 into master will not change coverage.
The diff coverage is 75.47%.

@@           Coverage Diff           @@
##           master     #636   +/-   ##
=======================================
  Coverage   68.53%   68.53%           
=======================================
  Files         368      368           
  Lines       40027    40027           
=======================================
  Hits        27434    27434           
  Misses      12593    12593

Impacted Files	Coverage Δ
include/cantera/base/xml.h	100% <ø> (ø)	⬆️
src/base/xml.cpp	72.8% <75.47%> (ø)	⬆️
src/transport/GasTransport.cpp	90.98% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d62f3b...172fa72. Read the comment docs.