What are names of the colon delimited fields in this FASTA header?
According to ensembl's website (albeit somewhat buried in a page about downloading data via FTP) it says:

"The header line in an FASTA dump files containing DNA sequence consists of the following attributes : coord_system:version:name:start:end:strand This coordinate-system string is used in the Ensembl API to retrieve slices with the SliceAdaptor."

A brief rundown of these attributes as I understand them:


coord_system- Is simply the coordinate system used to locate a particular feature within an assembly: "chromosome" and "BAC clone" are common examples as described here.
version- This is the current version of the assembly reference. And refers to an Ensembl archive as detailed here. (Yours appears to be from the human GRch38 assembly).
name- This seems pretty self explanatory. In this case we are looking at chromosome 21.
start- Again, self explanatory. Any complete chromosome or contiguous sequence is going to start at "1".
end- End of the fasta sequence. [start] - [end] + 1 = total length
strand- This refers to orientation (i.e. forward or reverse strand) 1 is forward (-1 is reverse)


Here is a link to a pdf containing this information and more about using Ensembl (see page 23)...

Unfortunately, to answer your final question, there is no easy way to come across this information and there are as many ways of labeling fasta files as there are sequence databases. Sequence repositories have come a long way in the last 10 years but there is still a lot of work to do toward standardizing data formats and making databases more user friendly.