[MRG+1] DOC insert spaces before colons in parameter lists (#7920)

* DOC insert spaces before colons in parameter lists

Complies with numpydoc to improve rendering and automatic quality
assurance such as #7793. Affects listings of Parameters Attributes,
Returns.

Performed with the help of:

    grep -nE '^(    )+[a-zA-Z][a-zA-Z0-9_]*: ' sklearn -R | grep -v -e
    externals -e tests | grep -v -e default: -e else: -e Warning: -e Note:
    -e TRAIN: -e Default: -e True: -e False: -e DOI: -e In: | gsed
    's|\([^:]*\):\([0-9]*\):\([^:]*\):\(.*\)|--- a/\1\n+++ b/\1\n@@ -\2,1
    +\2,1 @@\n-\3:\4\n+\3 :\4|' | git apply --unidiff-zero -

* DOC fix numpydoc format for param